Retention settings for individual suppliers and contractors are found within the properties for the respective contact within the Accounts Payable module (Payment Details tab).
By default, these settings will be applied to any transactions or commitments made for that supplier, however users can override the percentages if required. Please note, the rules are only applied to invoices when they have been created with the “Unlinked” option i.e. not when an invoice has been created from a purchase order etc.