They are both consultants.
The Expert track ones work on project workstreams relevant to their expertise (e.g. Digital in this case)
The Generalists can be staffed on a variety of workstreams
In terms of career progression - both have similar style of positions but different naming conventions.
Happy to give you more details on a question-by-question basis over DM.
However, if by Digital practice you mean anything in BCG Digital Ventures, BCG Platinion, or BCG GAMMA - then thats a completely different story altogether. The above answer is for regular BCG only.