$MX or submission credits can be used in 2 ways:

To get a discount on the amount payable for your current order. For e.g., if your order value is $50 and you have $MX 10 in your account, you could click on the 'Use $MX link' on the payment page and enter the entire amount of $MX 10 that you have available, this will automatically be used to reduce your amount payable to $40.

To pay for an entire order. For e.g., if you have a balance of $MX 14 in your account, you could pay for up to 100 directory submissions (calculated at 14 cents per submission) by going to the 'Submit your Sites' section and entering '100' in the 'Specific number of directories to be submitted' column, the entire $MX available would then automatically be used to pay for your order without you having to make any payment.

$MX 1 = $(USD) 1