Interline travel / ID90 tickets
 

Can someone explain to me how interline travel works pls? I'm a Thomson FO, and staff travel have told me they can usually negotiate discounts with most airlines, even if they don't have an existing agreement in place.
My question is, do most airlines offer ID90 tickets to other airline employees, or am I looking at an ID75 at best? Also, are ID tickets class-specific, and if not, are some airlines better known for upgrading us than others...and if so, which? What about SQ and VS?