The mduw has a working memory that holds a bunch of patterns and samples.
Back before the plus drive, this working memory was all you had to work with at a time: The patterns in slots A01-H16 and up to 48 .wav samples that could add up to no more than 2.5 MB.
This meant if you wanted more, the only option was to replace some or all of the current contents of the working memory. You could save anything – from just a single pattern or sample, up to the entire contents of your working memory – as a sysex file to your desktop, preserving it as a picture that could be reloaded later.
This picture had a size limit – the size of the working memory.
Your computer could store a virtually unlimited number of these picture files, a nice advantage. The downside: you still need a computer a lot if you make a lot of patterns.
So Elektron created plus drives. Instead of the unlimited number of picture files your desktop can hold, you can store up to 128 of them right aboard your mduw+. They called the picture files “snapshots”.
Same idea, same size limit.