Basics of Account receivables account, separate, deposits, discount, restricted

Jupiter

Please clarify followings
thanks

1. Is sinking fund related deposits are part of restricted cash?
2. Is Sales/cash discount account is contra account to revenue?



3. Is Bad debt expense a separate expenses account or it is contra sales account?

4. When the actual (realized) bad debt expenses are less than anticipated (and recorded as Allowance for uncollectible) how is the gain recorded on Income statement? (is it by crediting the amount to bad debt expenses account or as a gain in other revenue?)