When you open a Selling or Trading thread, someone from staff will contact you for proofs depending on the trackers you list. Submitting proofs for Selling or Trading is compulsory and if denied, you will be considered a high risk user and will be banned.
When applying for a giveaway, the OP might ask for proofs in order to decide if you are a good user or not. Providing proofs here is optional and the OP can decide if he still wants to invite you or not.
Click Here for a Tutorial on 'How to Make Ratio Proof, Speedtest and Seedbox Proof'