What is a digital platform? In context of European research projects, they invest into development of digital platforms.

Here is a source to give a definition  but not really, and every time people refer directly or indirectly to Apple, Google and Facebook. 

Somehow, they try to replicate the business model, but not culture.

But, the question is: is there a formal definition of what digital platform is?
I am not aware of any definition, and believe me, lots of people would like to have one - mostly for regulatory purposes. There are attempts in many countries to regulate digital platforms, but without good definition, you can't create a good legal regulation.

Any definition I have seen so far is a list of features, that is, however, not exhaustive - it would be a good definition if we could say, "it is a platform if it has those 8 features," but then you will find one that doesn't have all of them, and you find something that has all of them, but you won't call it a platform...