rehitnama

I've just come across Bhai Nand Lal's Rehitnama, a conversation exchange he had with Guru Gobind Singh ji. But I have come across three different versions, quite different from each other and one of them even says something along the lines a Sikh shouldn't sing anything other Guru's gurbani or...

Pray Truth for all and say Satsriakal! Dear all! May I ask who is a Rehit Naamee Sikh? Is he one who follows the Rehit Naamaa? Balbir Singh