When talking about "mere aggregation" vs "larger work", i.e. if a work under license X should be considered a separate program or a part of the larger work, GNU GPL license and its FAQ section go deep defining the difference:
Where's the line between two separate programs, and one program with two parts? This is a legal question, which ultimately judges will decide. We believe that a proper criterion depends both on the mechanism of communication (exec, pipes, rpc, function calls within a shared address space, etc.) and the semantics of the communication (what kinds of information are interchanged).
If the modules are included in the same executable file, they are definitely combined in one program. If modules are designed to run linked together in a shared address space, that almost surely means combining them into one program.
By contrast, pipes, sockets and command-line arguments are communication mechanisms normally used between two separate programs. So when they are used for communication, the modules normally are separate programs. But if the semantics of the communication are intimate enough, exchanging complex internal data structures, that too could be a basis to consider the two parts as combined into a larger program.
This basically means that if semantic of the communication between, say, backend software and a frontend software is intimate enough (see an example here) they may be considered as one larger work, and distribution of the frontend part would mean the distribution of the whole.
I've found nothing related to "mere aggregation" vs "larger work" in other copyleft licenses and their FAQs.
So I wonder if the principle described in GPL FAQ is something related to GPL-family of licenses only or it may be considered universal and can be applied to, say, MPL too?
Could it be that a privately run backend service containing
Mozilla Public License dependency is considered distributed ("conveyed") to users if the frontend part is distributed and semantics of the communication between the backend and the frontend falls under "intimate enough"?