Well, service history of vehicles here in "Dispatch Forum" or here - https://imhd.sk/ke/vehicles-in-service is totally incomplete, because it's made by observation of transport fans (depending on their free time).
That's why it seems that lot of vehicles don't run very often, but that's not true for lot of them.
An exception from the trams is, for example, tram no. 701 (Pragoimex VarioLFR.S), which did not operate for a very long time due to traffic restrictions due to COVID-19, but otherwise by default is sent to one specific duty serving most of the time routes to U. S. Steel.
Vehicles that aren’t in service very often.
2TheMHDmarek3’s previous post lists:
Pondelok 17.5.2021
6: #528
9: #501, #500.
According to their service history, they don’t run very often. But by the time 2TheMHDmarek3’s posted, they were back in the depot for who knows how long.
2TheMHDmarek3
6: #822
9: #827
10: #4733, #4736
17: #4725, #4765
32: #5725
72: #4708
-----
Nedeľa 23.5.2021
6: #825, #835
9: #814, #845
17: #4747
36: #4711, #4733
71: #4704, #4708, #4741
-------
Pondelok 24.5.2021
6: #610 + #611
9: #508, #531
17: #4752
71: #4723
-------
Utorok 25.5.2021
6: #820
9: #508, #512, #521, #535, #537