Menses autocomplete is a feature that automatically fills in the number of menses days based on your previous history. So if you had 4 day long menses in the previous cycles then this feature will automatically fill in 4 Menses values for your current cycle. If the predicted number of days is different from the reality then you can always change it manually by adjusting the menses symptom.
If you like better to set the Menses symptom manually every day then you can turn off this feature in the Preferences:
(menu -> Preferences -> Menses autocomplete)