Detailed documentation on RGB Combination

There's no official one yet, we're working on releasing a good manual with the 2.0 stable release. That said, there's a lot of experimentation in the combine tool that no manual really would cover, it's very personal preference in that sense. If you start with the preset formula's (and press "new formula") you have the basic "best" starting point. From there you can adjust the values and I would just experiment with those. Here's a post in which Mabula explains how to use the tool for better star colors: https://www.astropixelprocessor.com/community/postid/25158/

Thanks a lot Vincent for your answer. Presets are really good and I usually use just them with modification.

That said your asnwer only partially answer my question on meaning of "L", "BG" and multiplier sliders. I will read the Mabula's post, but really look forward for a comprehensive user guide (I am old school, and like book/booklet/reference guides).

Thank you,

   Alvise