It's common for the number of streams available simultaneously from a given IPTV provider to be limited to a small number (typically two to four). Thus the ability to limit the number of simultaneous recordings on a given custom channel source to that number is essential.
However, it's not uncommon for an M3U to be organized into groups of channels from different countries, and that doesn't jibe too well with having only a single default guide lineup for the entire file. Imagine you've pruned the provider's list down to just US, Canadian, and UK channels. What should be the default guide lineup? While it's possible to temporarily add additional guide lineups while editing the channel lineup, it's a PITA, and I'm guessing it's not well-tested.
I'd like to suggest that each separate 'group' in an M3U Plus file be given its own default lineup. M3U Plus files are often grouped by geography, and so it seems logical to align a group representing a country with a default channel lineup for that country.
Before anyone suggests that 'just split the M3U into one per country', that would effectively remove the cap on the number of simultaneous recordings. Another approach to solve the issue would be to allow a group of M3U sources to share a cap on the number of simultaneous recordings. That may be simpler to implement, and still meets the same goal.