The purpose of the Payment Source field is to determine weather a payment has come from a patient or a health fund, and to allow filtering payments in reports by their source. Its is linked to the payment type in that when you create payments of certain types we set the source by default.
For payments of the type Credit Card, Portal, Cash and Bpay we default the Payment Source to Patient.
For the payment types of Online (Eclipse payments from funds and electronic payments from DVA and Medicare) Payment Source defaults to Fund
In all other cases it defaults to Unspecified.
Other is really for use by the user, to group payments you want to keep an eye on or don't yet know what category it belongs in.