Ok this question is hard to answer so i’ll try to make it simple as possible.
Every character is given a budget of :
- 1 Aspect
- 2 Stunts
- 6 Skill levels
TO BUY extra only !
The budget :
The first thing to understand is that this “extra budget” is like a currency. It’s not 1 Aspect, 2 Stunts and 6 Skill levels to develop his/her character per se. This currency is only for buying extras. Simple.
But extras are described in this game as Aspects +/- Stunts +/- Skill levels you see ? Each extra are a combinaison of the three “currency”. That why they cost Aspects+Stunts+Skill level to buy. Generaly cheaper than the combinaison of them.
You cannot use this extra budget to buy stuff for your character other than “extra stuff” but the opposite is true. If you miss 1+ aspect or stunt or skill level to buy an extra you can take it from the “normal” aspect/stunt/skill level allocate to create your character.
What extras can I buy ? :
You can buy any extra you can/want BUT some are restricted. Restricted in the sense that they must/may be included/listed as extras in a Culture or Genotype or Occupation or Organisation. If the character gained/paid one of those condition/restriction during creation (or advancement later) he/she is open to take the Restricted Extras.
As example, if during your campaign, the character is welcomed/associated in a culture/organisation/genotype/occupation (in one word restriction criteria) that is not his/hers during creation and can justify a “cost” (in any way as in RP for instance) he can now “buy” those extras with his/her advancement later on.
Others restrictions are : Tech Index, Genotype, Culture, Organisations or Occupations or what ever you as a GM need them to be.
What to pay :
When buyins an extra you can buy ANY part of the cost you want. You just gain that part of course. If an extra cost 2 aspects, 1 stunt and 1+ skills. What ever that mean is explained in the description of this extra. Whatever you pay, you get. The rest could be bought next avandcement.
And then there is “teamwork bonuses from extras”, Shared extras, Damage to extras to read in the book
Hope that is clearer as i don’t use english as my natural langage