Assume there are three maps in the mapset - MAP1, MAP2 & MAP3. Assume MAP1 requires 100 bytes, MAP2 requires 200 bytes, MAP3 requires 300 bytes of memory. If you sepcify STORAGE=AUTO, memory acquired the mapset is 600 bytes. i.e each map will have it's own area.
If you don't speciyf AUTO, it will be defaulted to BASE. In that case, mapset will acquire only 300 bytes (max of the maps in the mapset). The same area is used by all maps. (kind of redefine.)
In the case of BASE, if your program moves some data into MAP1 and later also into MAP3, content of MAP1 are lost (over written). In the case of AUTO, it does not happen.