Thanks Dr. Sassi. I tried setting the generate to spline but it didn't work. The alembic asset doesn't have an alembic selection tag in C4D.
This is what I did: created hair > selected some of the hairs > set selection (name the tag selection_1) > inverted the selection > set the new selection to a new tag (name the tag selection_2) > made two hair materials (feathers_1 and feathers_2) > apply the two materials to the hair object > in the tag of the materials selected Use selection and apply Selection_1 and _2 to each > just in case, I also made two standard materials and apply them to the hair object and to each of the hair selection tag names > in the hair object (Generat > Type: Spline > I tried selecting and deselecting Render Hairs) > I tried both with bake as alembic (on the right click menu on the hair object )and exporting it as an alembic
In any of these cases I got an alembic asset with a selection tag. The only thing that was exported in the alembic asset was the two non-hair materials but with no selection tags, so the materials were doing nothing.
I also tried exporting more than one hair object inside a null, or a connected object but the hair objects don't export a alembic if they are inside another object.
Any help will be very welcome. This workflow for exporting hair with selection tags is needed to be able to create hair in game engines, and I guess to export hair to any other application. Thanks.