A Masterwallet is a project-unique user account wallet which users are able to perform direct deposits or withdrawals. All deposits into Subwallets are also summed up and reflected in the Masterwallet's total balance.
Some merchants may require additional funds to be deposited directly as they use Multichain for its mass withdrawal function. This is true where the total sum to be withdrawn (mass withdrawal) may be insufficient.
Subwallets are generated thru Multichain's developer API (Create Subwallet) where it can be assigned to every unique user on your platform indefinitely (does not expire) and be notified when a deposit is received. All funds received are then reflected in your Masterwallet's "Total Balance", available for withdrawal.