The following are notes from a conversation I had in chat. I hope to expand it some more as I delve into this more.

In webgui there are two subscription concepts in WebGUI

1. Subscription Sku asset - which is used (it is a wobject iwth connection to the event manager). This adds you to a group when you pay. used for selling access to some area of the website.
2. There is a "subscribe" to some function, which sends you an email. This is typically used in the bazaar, CS or Newsletter functions.

Both just add you to some group, so they are kind of the same concept, the difference being that case #1 you purchase the subscription, and in case #2 the subscription is Free.

The newsletter inherits from Collaboration assets.

in the db schema, the subscription table is for the subscription sku. - it stores the subscription properites of the thing you are trying to sell.

The actual subscriptions (for both types) are stored in groups, but these groups are not seen in the admin console.

Now there is the  AssetAspect::Subscribeable. WikiPages and the WikiMaster have this.  But you can extend any existing asset to incorporate the AssetAspect