Help needed on tables

Hi all ,

I have following questions , can someone give me brief details on these

1. What is the meaning of secondary index tables? for example what is the difference between BSIS and BSAS..

Some sample tables for my question are--

u2022 BSID Accounting: Secondary Index for Customers

u2022 BSAD Accounting: Secondary Index for Customers (Cleared Items)

u2022 BSIK Accounting: Secondary Index for Vendors

u2022 BSAK Accounting: Secondary Index for Vendors (Cleared Items)

u2022 BSIS Accounting: Secondary Index for G/L Accounts

u2022 BSAS Accounting: Secondary Index for G/L Accts (Cleared Items)

2. Who is a 1099 vendor? what's the meaning of 1099?

Thanks