In latest DJI GO 4, in General Settings there is a switch called "Clear Cache Automatically." I do understand its function.

In General Settings there is also a setting "Maximum Video Cache Capacity" with a selection of values from 2GB to 16 GB (these I also understand).  However, in the same list there is a selection called "Auto." What does "Auto" mean here?

Never mind - answer is here:

marlowe Posted at 2018-1-20 18:35
Never mind - answer is here:
https://www.youtube.com/watch?v=EvkhIKAfMSc

Glad you find the answer. If you find your phone's capacity is not enough, you can enable the feature to help you manage the cache files.

Yeah, be careful setting auto as space gets filled quickly then footage will be auto deleted when your device storage is maxed out. I find setting 16GB easier to manage.