Hi Nick ,
for FFF please refer below link
http://help.sap.com/saphelp_em70/helpdata/en/ed/3e3842cd38f83ae10000000a1550b0/content.htm
RBA - (Rule Based ATP Check )
It states the rules using which you can substitute products , locations , PPMs etc .
In Product Substitution you define rules i.e product A can be substituted by Product B and validity of the rule . You can have a look into /SAPAPO/RBA04 .
In product interchangeability you state that demands for a product A can be fulfilled by using Product B after a certain date . (After which the product A is considered to be not valid any more for customer demands). Its like you are replacing the product A by product B .
There two scenarios related to interchangeability
1- Super session
2- Form Fit Function Classes
I advice you to please go through below link .
http://help.sap.com/saphelp_scm41/helpdata/en/ad/e90bdc9b619f47af80f8b0c1b5b434/content.htm
please let us know in case of any further questions on the topic
Best Regards ,
Love Singh