Always Keep All BIA Index Data in Main Store (Keep in main memory)

Hi:

Need clarification about the BIA Index Property "Always Keep All BIA Index Data in Main Store (keep in main memory)":

1/ What is the difference between BIA indexes that use that settings (vs those that do not)? My understanding is that all BIA indexes are in BIA server memory already.

2/ What are the risks and limitations of using that setting e.g. BIA overload.

3/ Are there significant query performance gain when using that setting

Thank you,

Catherine