A cassette with a universal fit fits several tablets within a certain range, e.g. between 9 and 11 inches.
Before the tablet can be put into use, extra work will have to be done once to position it properly with fasteners.
In some cases, a transfer frame will also have to be used.
The advantage of the universal cassette is that multiple tablets can therefore be placed in it. Easy if a newer tablet is purchased in the future, for example.
In general, universal cassettes are also cheaper.

A cassette with a customised fit is only suitable for the indicated tablet(s).
The advantage of this is that the tablet fits seamlessly.
However, if in the future another tablet with different sizes is used, the tablet may no longer fit in the cassette and a new one has to be bought.