When you set "Optimize for Time Machine" option enabled in Virtual Machine Configuration, snapshots are being taken every 48 hours.
The last 3 snapshots are being kept and Time Machine is backing up only the last snapshot, but not the whole Virtual Machine. This allows you to reduce the amount of data that Time Machine backs up, to reduce the time Time Machine spends backing up the virtual hard disk(s).
on Parallels Desktop version 8:
on Parallels Desktop versions 9 and 10:
NOTE: If your Time Machine is backing up the whole Virtual Machine file while Optimize for Time Machine option is enabled - it is a known misbehavior for Parallels Desktop 8. This issue was fixed in Parallels Desktop 9