If a Data extension in Marketing Cloud is marked as Sendable Data Extension, does this mean all the records within this sendable data extension will be calculated as billable contacts?
No it doesnt. The billable contacts are contacts on the all contact list. By definition , a sendable data extension is a data extension that has a column mapped to the All Subscriber List. e.g if you have a ContactId that represents that SubscriberKey in that data extension, you would have to map it to SubscriberKey so that system can resolve it at send time.
Otherwise , the Data extension is just a data store.
Response to additional questions in comments:
- Are all subscribers part of all contact? Yes they are. Susbcribers count as email studio audiences, while contacts are a more broad category (think omni channel, sms, push , etc)
- You mentioned that subscriber key needs to be used in sendable data extension for system to start sending, what kind of send activities are supported?: Yes for emails, you can use any subscriberkey (a word of caution, best practice dictates that you use an externel system id (eg. SalesforceID) if its integrated with it. In addition to the if you dont use the 18 digit SFID as your subscriberKey then tracking from your sends will not be sent back. For SMS, the subscriberkey will be the mobile number yo of the subscriber.
- What about customised send activities? Do you mean custom activities in Journey Builder? If yes these will be depend on which channel your are send off. It is import to think about the channels. What you use as subscriber/contact key will depend on the channel you are sending from.
This question might should shed further light on this topic : What is the difference between Primary Key Contact ID, Contact Key, Subscriber Key and Subscriber ID?