The term UDG might be specific to Spectrum, but quite a few computers were designed to allow other parts of the memory to be used for the character set. On the Vic (and indeed the Spectrum) the firmware system variables have an address that holds a pointer (another address) to the character set bitmaps. This by default resides in ROM, but if a user changes that value, they can then use the new memory to POKE values in to create UDGS.
The spectrum makes it more usable by having the UDG statement and the user of data, so that users don't have to remember and change address values. I don't think many of the 8 bits had that. But as I said, you can redirect the whole character set to somewhere in RAM and recreate the whole character set from BASIC.
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