Evening, gang.
Following a request from @mikeslr , I'm repeating this information here.
Basically, it's advice to use Qt packages as a 'loadable' SFS rather than installing Qt permanently through the PPM. Qt is almost universal these days, and is increasingly used in nearly all software to a greater or lesser degree. It's also updated very frequently, so it's quite possible that many packages that you can run on any given Puppy will often be built to run under different versions of Qt.
This leads to "issues", because if you try to have more than one build from the same series "active" at the same time, any app that needs to use modules from a version of that series WILL throw a tantrum, stamp its little foot, sulk, and refuse to come out and play..... 
This applies to all series of Qt. Qt3 (very old now), Qt4, Qt5, and the recently-introduced Qt6.....all display the same behaviour. And newer software that requires Qt to be present on the system is far fussier about having the correct build of Qt available. It's no longer as 'forgiving', and backwards-compatible the way that Qt3 or Qt4 used to be.
This is down to the fact that, for any given "series", the 'soname' module sym-link name remains the same, regardless of the specific version of that module. And as is well known, Linux doesn't like two files with exactly the same name in the same location....
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If you load Qt (4,5 or 6) as an SFS package, you can at least maintain multiple builds on the system by the simple expedient of loading the appropriate build for the app you wish to run..... It's not a perfect solution - with Qt, precious few methods can truly be called 'acceptable'! - but it IS a 'workable' solution (at least for us here in Puppyland). Even if it IS a bit 'fiddly'!
Mike. 
