They're stored in the Ignition internal database, like your windows, groups, settings, etc.
No, it's not require- but then things like what you're trying to do won't work. The default database for an internal provider means that when something needs to access the database, and it doesn't specify a name, this is the one it goes to.
Yes, as I mentioned in the last post, this should be possible, but currently isn't. The reason is because back in FactorySQL/FactoryPMI days, the tags weren't aware of the different database configurations available, so DB Tags set to query mode only operated against the database that contained the tags. With an internal provider, you don't have a database- thus we've added the "default database" option. I think shortly, though, we'll improve the system and make this possible.
No, what I was trying to say is that there is in fact an expression function by that name. It's not in the drop down list (<-fixed for 7.2, though). There is a similar function in scripting, and your correct that runScript could be used for that, but that's not what I'm talking about. You could literally copy the code block I posted and paste it into an expression and it would work.
All in all, I think I just realized a big point of confusion for you- the misleading name "DB Tag". It's really not all that accurate, as only thing "DB" about it is that it can be set to query mode and run a query. It can also be set to expression mode, in which case it has nothing to do with the database, and it can be set to neither query or expression ("static mode") in which case it's just a static variable. On a side note, this static mode is the only mode in which you can write to it.
Even the name "SQLTags" is a little misleading now that we have the internal provider. But, we didn't think it was a good idea to change it when we made Ignition, so that's how it is.
Hopes this clears things up a bit,