2 sets of what? qualifying or race tyres?( you can use both sets of tyres
in practice and qualifying sessions, but you can only use race tyres in a
race). NB :- you have 3 sets of qualifying, and 4 sets of race tyres.
If you have not used a particular set of qualifying tyres, you will not
be allowed to use them during a race, only race tyres can be used in a
race.
When? during a race, when you pit, you are given a new set of tyres, or
if they have all been used, you are given the set which is least worn.
During a practice, warm-up, or qualifying session, you can choose which
set you want to fit, eg:- any qualifying or race set.
d:~))
**Peter**