Scheev

With appdata its typically a good idea to keep it on the cache drive because an ssd/nvme is more advantageous due to their speed for docker containers, most other things could do without the speed of an SSD.
 
With that said, when defining a share, you can choose its "primary" and "secondary" storage. Using appdata as an example, if you intend for it to exclusively use the SSD as its storage you would select "Cache" for primary storage and leave it at that. But in the case where you think your cache drive may fill up, you can also set a "secondary" storage. You would set this to "Array". 
 
This means that if the cache were to be filled, unraid would start placing any new files onto the Array instead of the cache drive. So this is where the "mover" comes in, the share will contain a field for "Mover action" if you have specified a primary and secondary storage. Essentially the mover will transfer files in the direction specified. In the case from before where we set cache as primary and array as secondary, if we specify "Array to Cache" this means that any files placed into the array from appdata will move back to the cache when the mover is invoked. However, that only happens if cache space has been freed up.
 
Hope that answers your question.