P6 User Preferences

Hello,

Could someone help me to understand what is the diffrence and when or how to use these two options given in user preferences -calculations resource assignments title ?

When adding or removing multiple resource assigments on activities

1- Preserve the units, duration and units/time for existing assignments

2- Recalculate the units, duration and units/time for existing assignments based on the activity duration type.

I know we select duration type when we assign an activity. But I did not understand why we are given options to select when we assign or remove multiple resource assigments on activities.

Thanks, Regards