I have used T-5 lamps in a multitude of undercounter fixtures, and in some troffer fixtures. These in the T-Bar fixtures are more known as PL type lamps. They come in T-5 and T-3 configurations.
The ballasts are typically electronic [High Frequency - High Power Factor], but can also be the typical magnetic [reactor core] ballast, used with electronic circuitry to form what is called a Hybrid type ballast.
Lots of compact fluorescent lamps are T-3 size.
It, of course, seems somewhat small- however when I first saw T-8 lamps being used in Commercial projects [circa 1993], it was very strange. Now it is strange to see a T-12 lamp anywhere
When I see one, they just look odd!
See everyone in a week, or so.