Custom Reports via BIRT and the RTC datawarehouse

Need an explanation as to the snapshots contained/generated by the data
warehouse within RTC. I don't know how to choose what snapshot for
what type of report or for the data contained within. I have found
some superficial information on the wiki, but it isn't sufficient or
detailed enough for me to make informed decisions as to which snapshot
to use for what. Where is there further instruction/information on
using the data warehouse for custom reporting via BIRT? It is a
question of how to use the data provided and what that data is
representing.