Rules? What rules?
For cognizant QRP practitioners, the 'rules' have been agreed to over the years by QRP clubs such as QRP-ARCI (after it changed from a QRP standard of 100W) and GQRP, to mention just two of the oldest most traditional QRP clubs, and them made 'official' by the long defunct WQF.
Others then added the terms Milliwatting/QRPp and Microwatting/QRPpp as QRP 'sub-divisions'.
As for ERP, RMS etc. power, if I recall correctly, the "standard" was chosen for simplicity and/or the assumed lack of accurate test measurement equipment in most ham shacks. Some may want to nit pick here and there, but there is no reason for it, as long as all QRPers follow the same 'rules' when bragging about their achievements.
That said, 5W with an 8 element monoband on a 100ft tower or 5W with a dummy load will still be QRP for operators with different outlooks.
Other "bones of contention" are the view that QRP is CW only and/or a poor man's version of amateur radio.
Let's not talk about the "Life is too short for QRP" snide.
