when I'm scheduling, M1 sometimes stops to show me a rule violation: Artist Period. What does that mean?
Artist Period is the DMCA rule. No more than 4 songs by sameartist within a 3 hour period. The other element of it is no more than 3 songs from the same album within a 3 hour period. the DMCA rules, not the stupidest thing congress has ever done, but brain-dead, all the same. So when you see Artist Period violation, it means you've already got 4 songs by that artist scheduled within then 3 hour window where M1 stopped for the violation.
If you set all Artists for a 1 hour separation (or more), then you shouldn't see that rule ever during normal scheduling runs. Because if the artists' songs are all separated by at least an hour, then certainly their can't be 4 songs by any artist within a three hour span.