Often times nodes need to communicate with each other what their implementation is, looking for specific features that are only in select implementations, the desired feature is a question of node implementation.

A concept of Unspecified protocol extensions has been proposed, they are unspecified as they are not part of the BOLT specifications, each extension must have a set of random 20-byte identifier with type bigger than a certain number. Using the concept of current extension, any extensions that are unidentified by a node will not be recognized, while if the node’s implementation identifies this unspecified extension, both nodes can continue communications knowing that they have the same set of extra features.

Another possible implementation here is having feature bits having 1s for the features your node can recognize and 0 otherwise, this will grow with the number of extra features supported and so the first implementation can be more plausible.

link to proposal