I believe "Plug & Play" objects are NER BSFNs wich contain calls for one or more localization objects for that specific transaction, depending on your country, etc.
At least every Plug & Play BSFN I saw, was. Not sure if it is allays true.
Anand,
Plug & Play are like exit points created by JDE to include localization for countries.
There are P&P in many Master Business Functions (A/B, A/P A/R, SO...) and avoid usualy are used by companies to avoid customizations into original applications. You get a break point and for example a new invoice is included.. so if country is = "IT" then default informations run a specific BSFN of italian localization...