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The Effective Date defines when a package 'takes effect' from a billing standpoint. Effective date is used as it relates to usage billing. For package billing however, once the package has billed once, it has no effect on the package billing itself going forward. It continues to effect usage billing periods
As of EngageIP 8.6.0 user-package effective dates have several restrictions to prevent invalid configurations and billing issues. User-package effective dates can no longer:
Be set to a future date
Be within a closed financial period
Posting Date
The Posting Date is a field that is used for reporting / accounting purposes and is supposed to reflect the date on which you want to recognize the revenue as opposed to the date it was added.