Just had a quick look into the file /var/ipfire/backup/include and saw those lines:
etc/sysconfig/*
etc/sysconfig/firewall.local
Am i correct assuming that the first line already saves firewall.local, too and the second line is obsolete?
Just asking 'cause I’m adding additional files using include.user and struggled about the correct syntax I’ve to use myself.
There is overlap in a couple different areas (e.g, rrd directory). I think it is mostly a “belt & suspenders” type thing but I honestly don’t know.
Anyway, I agree and I think the second line is not needed. But the only way to find out is to give it a try!
There is a small section on the wiki the might help:
Side note to others: I suggest not making any changes to the /var/ipfire/backup/include file. Future core update might overwrite the include file.
For the records 
I did not change this systemfile but the user one.
Nevertheless, thanks for your answer.
OTH, I added /etc/nut/ to include.user and no files where included in the backup ISO. However there is a dedicated backup button for nut in WebIf, that will do the job.
Any reason why my custom setting did not backup those nut folder?
Are you running Core 169?
Several releases ago, the backup “excludes” would supersede (override) any “includes”, but that doesn’t seem to be the case anymore. Also, the syntax for includes and excludes omit the beginning “/” for paths these days.
Still on 167. And I do just use include.user and not exclude.user. But anyway, since nut settings get backed up in another place, all is well by now.