The date proposal specifies a sequence of dates which can be used during order processing as a reference for date determination. The date proposal is only relevant for the billing plan type Milestone billing (periodic billing = blank).
When you create a billing plan with milestone billing, the dates
are copied according to the reference, redetermined on the basis of
the current rules, and placed in the billing plan.
To maintain the date proposal for milestone billing, go to the detail screen of the date proposal and select the function 'Maintain dates'.