Halliday & Matthiessen (2014: 489):
A non-finite clause, on the other hand, is by its nature dependent, simply by virtue of being non-finite. It typically occurs, therefore, without any other explicit marker of its dependent status. Hence when a non-finite clause occurs without a conjunction, there is no doubt about its hypotactic relation in a clause complex; but there may be no indication of its logical-semantic function. …
The best solution here is to find the nearest finite agnate clause. If this is a non-defining relative clause, the non-finite is elaborating. If it is a coordinate clause, the non-finite is extending. If it is an enhancing clause, the non-finite is enhancing and could probably be introduced by a conjunctive preposition.