I just found out that Grimshaw’s Element.GetFromLinkedFile (in Archi-lab) works only on the first linked model, but others. Could anyone please guide me to modify its python to make it work for all linked models.
Thank you very much
Thank you very much for your quick response. I know this method but I like to modify it and make it run automatically on all linked models at once. 
Then what is the result you’re looking for? A combined list of all revit links or a combined list of all categories from all links?
I am looking for the result of a combined element list from a certain categories taken from all linked models.
For example: There are 3 pipes, 4 pipes, 5 pipes in the linked model 1, linked model 2 and linked model 3, respectively. The result of the combined list should be 3+4+5 = 12pipes.
Thank you