VESA is a standard for mounting displays on varies surfaces, stands, or arms.
It is relevant for pen displays because pen displays are essentially monitors, and many people want to mount them similarly to a monitor.
Not all pen displays support VESA mounting
13" and smaller - very rare that they support VESA
16" - Most do not. But a few do.
20"+ - Very common. As far as I know all pen displays at 20"+ support VESA.
Basically VESA support means there will be 4 holes in a rectangular pattern on the back of the pen display. The distance between the holes corresponds to how big/heavy a display it is intended to support.
The hole patterns are usually specified by the distance between the screens. For example: 100x100mm. These hole patterns also name names like "FDMI MIS-D" but nobody uses those names.
FDMI MIS-B
50 x 20 mm
M4 x 6 mm
FDMI MIS-C
75 x 35 mm
M4 x 8 mm
FDMI MIS-D
75 x 75 or 100 x 100 mm
M4 x 10 mm
FDMI MIS-E
200 x 100 mm
M4 x 10 mm
FDMI MIS-F
200 mm increments
M6 x 8 mm or 10 mm
M8 x 15 mm
For pen displays, in my experience the most common pattern is 100x100mm.
Not that the different hole patterns use different screws. For 100x100mm pattern, the screws are M4 x 10 mm. And specific devices may use diferent screws in some cases such as FDMI MIS-F. If you are not sure, contact the manuacturer of your tablet for screw size information.