"Deposit in use" is the part of your deposit that is currently covering your current balance.
A Quick Example
Total deposit: $1,000
Current balance: $600
Deposit in use: $600
Available deposit: $400 (Can be used for paying rent, bills, etc.)
Paying Your Balance
If you pay from an external bank account:
We withdraw $600 from the bank. The $600 hold on your deposit is lifted, so your available deposit goes back up to $1,000.
If you pay from the deposit:
We move $600 out of your deposit. Your new total deposit is $400, which is also your available deposit.