Hierarchical, I think, excellent (I'd like the option for every site). Different tag sets, of course (by basics of tagging I meant to exclude actual tags, merely include "the system"). Meta's tags are mainly used as a way for the site builders and administrators to keep track of what is completed - for the moment, later they will be important to build up the culture specific to sites. Disregard the importance for general users and you are setting a bomb with a delayed fuse.

Other sites may not want mod-only tags damns the rationale for having them on other sites. I have never seen much need for mandatory tags, but to demand them on one site and not another makes no sense to me. If it makes no sense to others there will be confusion, and errors, and extra work and ill feelings. Implementation of a trust level system (changing who counts as a 'mod' for 'mod-only' tags) is irrelevant to my point, which is cross site consistency in the basics

ie some users can and some can't, even if the qualification system varies between sites. Lower case or upper or mixed is fine, but mixed means more work, both manual and very likely automatic (so slower). Being limited for character number was not my point – my point was the limit should be the same for all sites (as a matter of course, rare exceptions allowed, as everywhere, with strong justification). By tag number limit I meant per question.

The symbol for imported is an example where I think "exceptional circumstances" applies. If there is a tag-quantity-per-question limit then that symbol should not count towards that limit. No spaces – good, maybe you will better understand what I am getting at. Would you mind if a site decided (without any particular reason, just a mild preference) that spaces be permitted?

Disallowed characters – similar to above. I don't mind whether or not there are any but, without good reason (eg all Greek characters disallowed in general but permitted on a language site) any exclusions should, unexceptionally, be the same for all sites.

Other sites may not want mod-only tags damns the rationale for having them on other sites. // Allowing the functionality to exist doesn't mean a community needs to use them. Part of the vision of Codidact is enabling communities to make the best decisions possible and use the best tools available ON THEIR SITES.

Would you mind if a site decided (without any particular reason, just a mild preference) that spaces be permitted? // Not at all. If those communities need that functionality and want it enabled they are absolutely allowed to make the best decisions for them.

By tag number limit I meant per question. // Again, needs to be decided by the community. Some communities may want to limit every question to one or two tags. Their prerogative. Other communities may allow a virtually unlimited number of tags. It's not the "Codidact team's" job to tell them the best way to manage their sites, tags, special characters, etc. We are here to enable them with the tools they need to make the best decisions on their own.

A big piece of the Codidact philosophy is that the communities - the people actively involved asking and answering questions on a specific topic - are more familiar with the best way to manage that process than some central gatekeeping authority. The "Codidact Team" may suggest best practices, provide administrative and development support, and enforce the ToS, but we aren't in the business of telling communities how they ought to be run.