I got help off list as response to posting. As far as I have
understood:

For the supplementary radiation groups 5xxxx (those that can follow
55SSS and 553SS) it is assumed that the value cannot be bigger than
5299 (in whatever unit). As after the 55SSS group there can follow
only 553xx (or bigger) main groups, all other smaller 5xxxx groups
must be supplementary radiation groups.

From all 6xxxx groups of section 3 only the last can be precipitation
and only if the indicator in section 1 says so (values 0 and 2); all
other 6xxxx must then be supplementary radiation groups.

Thanks!