I presume you mean the 'a' variant ...
Your understanding is correct -- services in an 'a'-variant library (for PICC and PICC-18) can be called from the foreground (ISR) or background (task, etc.) levels.
The complication that arises with the PICC and PICC-18 compilers comes from the #interrupt_level pragma and how we use it to generate libraries. You'll need to review the end of AN-1 and all of AN-9 to fully understand how to call Salvo services from within interrupts with these compilers.
[This message has been edited by aek (edited July 24, 2002).]