No idea on RFSU... Google turned up all kinds of interesting links... maybe 'radio frequency syncronization unit'? Or 'Swedish Association for Sexuality Education'? (No I'm not making that up.)
A/G stands for above-ground piping. This is usually connected to some U/G (under-ground) piping.
If these acronyms are listed in drawings or specs for the project, I'd ask the engineers to please include a symbology sheet listing all acronyms and drawing symbols.