The simplest explanation is that they are regular ALPP shares that can't be traded for a couple years.
A longer explanation - If you have the class C shares, they can't be traded for a couple years, convert to class A shares when the restriction expires, would receive dividends if they were to be issued and would get a share of a pie if ALPP was to be purchased before the restriction is lifted. There are some voting differences which are not material to retail holders like you and I.
To be clear, the class C dividend is the one that occurred last summer. You can't get any more class C restricted shares. The prospective dividend in class D they recently mentioned will be structured similarly. I would imagine the only difference would be the timing of when the restriction would come off - the class D shares will be restricted for 2 or 3 years from the date at which they are ultimately issued (they would be tied to the issue date of the class C shares).
Hope that helps.