As already stated by Alisdair HERE, the dark eye provides some rules in addition to the base ruleset, called optional and focus rules. The former are meant as optional ways to rule certain aspects of the game at your groups gaming table. To work as intended you should all agree if you want to use those or not.
For example, using the skill-group checks instead of individual checks for each skill simplifies the game a bit, you will quickly know what attributes are linked to everything physical, related to nature or knowledge. But since this makes these attributes very important for ALL skills of the skill-group, it would change the balance oft he game if not all players use the same attributes for the same skills.
Another optional rule would be critical fumble tables for your attacks/defenses/spells/chants. Not everyone likes the idea of their character losing their weapon or botching their spells. So in the name of fairness, you should all agree if you want to use those or not.
Same for lower/higher parry scores or lower/higher regeneration of lifepoints.
Focus Rules on the other hand are not automatically intended to be used by everyone in your group.
Each player can decide when, where and if they want to „zoom in“ and increase the complexity of an aspect of the game.
One player might be totally fine with havin one roll determin how many rations his character can gather for the party by going hunting or foraging. They just inform the GM what they want to do, roll the dice and get a number in return, story continues.
But maybe hunting is a central aspect of this particular player character, or it isnt just any amount of abstract „ration“ they are hunting for.
In those moments you can „Zoom in“ break out the focus rules and get into as much detail as you want to track, identify, hunt and disassemble into usable materials for crafting or selling.
Both players can be at the same table. One contributes 3 rations, the other returns from a hunt involving multiple skills, with a wild boar worth 3 rations, a thick hide, some tusks and new skar on their leg.
While a Mage or wandering Artisan might be ok wielding a „staff“ to defend themselves, martial characters might be interested in the subtle differences of longswords from different places in the world, finding a weapon that really fits their fighting style. That might be worth a travel to Andergast, were you can get a „sword“ or a „andergastian sword“ if you chose to use the focus rules for weapons, which add a unique advantage and disadvantage to each weapon or armor oft he region/culture.
Other players wont really lose or win anything by not using this focus rule, except not having to remember 2 unique abilities, so it really is up to everyone of them.
Same goes for Hit Zones. You can wear „chainmail“ and subtract 4 points of damage everytime you are hit.
Or you piecemeal (or piecemail) together your own unique armor set and roll where you have been hit every time you take damage, it is up to your preference of how complex and detailed you want your game.
Each player can shape the game mechanics up to some degree, just following their own preferences.
House Rules are by definition not a part of the official rule set, but i think still worth to be mentioned. Almost every group playing table top roleplaying games will at some point add their own rules and thats obviously fine and intended. Here i just want to add a few quite common ones for TDE 5 Fate Points: RAW you get your Fate point maximum at the start of every adventure, and regenerate one by doing great deeds or completing important, epic scenes. If you want your game to be more heroic, and reduce the time players might struggle to decide if they want to take a risk, just refresh their pool of Fate points every session. They are meant to be spend after all. At my table we also add another pool of points (1 per player at the table) which players can use to reward actions they deemed to be athmospheric, cool or just entertaining. Additional defenses: RAW NPCs usually only have a single defense, while PC can dodge/parry multiple times, with each time getting higher penalties. We prefer an even playing field and allow every character multiple defense attempts. This does make combats a bit tougher. Low defense values: Some Creatures (especially mindless undead) have very low defense scores. Rolling a D20 for every zombie to see if they get a 2 or lower does add up in time. I prefer to just let the players hit and maybe add a zombie more or five. Makes games a bit more epic and combat more fluent.
Limited (Dis)Advantages: Usually you are limited to 80 Points worth of (Dis)Advantages. It is totally up to you as a group to just ignore this maximum. Especially characters that come with a lot of build in advantages, like elves, will be happy to still be able to customize their character more.
Still it is advisable for beginners not to overload their character with too many good or bad traits they need to remember.
Money: Every Player Character starts with 750 silver thalers and can buy his equipment with that money. Or you dont bother to calculate all that and just take what feels right.
Additionally you dont really need to pay every drink and meal with fictional coinage. If that kind of micromanagement isnt your cup of tea, just ignore it. I usually play a style of „your rewards are X amount of gold plus whatever your day to day cost is , dont bother“ just important purchases are really noted.
If you are not agreeing on this , there is a way to make everyone happy by some players buying a livestyle, either by paying a flat amount of money per month, or taking it as advantage representing a certain income.
Critically successful skill checks: Rolling a double or tripple 1 on a skillcheck is an automatic success and usually the best outcome your GM can think of. To make it a bit more significant you could allow the player to increase the skill by one immediately for free.
To keep things fair the rest oft he group can get the appropriate amount of points to improve their characters. While it might seem unfair that the lucky player doesnt get to decide what to improve, he will be on the other side of the deal soon enough. And the cheers of his party when he rolled them a few extra AP definetely is worth it.