IPC-4553 details the requirements for immersion silver (ImAg) final finish. 

The specification calls out thicknesses for Thin Immersion Silver and Thick Immersion Silver deposits.   

Thin immersion silver should be 2 µin minimum, while thick immersion silver should be 5 µin minimum. 

If there is no requirement/preference between thick or thin immersion silver, we would suggest the note to merely read "Immersion silver final finish per IPC-4553."