Fusion can take snapshots of the state of a virtual machine automatically at set intervals, with the AutoProtect function. This function is in addition to manual snapshots, which you can take at any time.
When you turn on AutoProtect, you set the maximum number of AutoProtect snapshots to keep at a time. After this number is reached, Fusion deletes the oldest AutoProtect snapshot in the range to make room for the newest snapshot.
This number setting can affect the number of manual snapshots that you can take and keep. You can have no more than 100 snapshots for a virtual machine, and depending on the types and sizes of the virtual disk files, this number can be less. If the total of AutoProtect snapshots reach the limit, you cannot take manual snapshots.
Fusion uses this setting to estimate the minimum amount of hard disk space the AutoProtect snapshots will use. That information appears at the bottom of the panel. The maximum amount of hard disk space can be significantly more than the estimate, depending on the amount of changes you make between snapshots.
When you have manual or AutoProtect snapshots of a virtual machine, you cannot add or modify its virtual hard disk.