what is meant by association with sigma or (p) factor 'alters the specificity of RNA polymerase"???
RNA polymerase is only capable of catalysing the process of elongation. It associates transiently with initiation-factor (σ) and termination-factor (ρ) to initiate and terminate the transcription. Association with these factors alter the specificity of the RNA polymerase to either initiate or terminate.
Initiation-factor (σ) or sigma is a protein and transcription initiation factor. It enable RNA polymerase binding to the promoter. Different types of sigma factors are specific for transcribing different genes.
A ρ factor (Rho factor) is a prokaryotic protein required to release the newly made (nascent) RNA from transcription complex.