The following article also applies to the other OM-D and Pen models but with less Fn buttons to choose as alternate AEL/AFL button.
Many photographers prefer to decouple exposure meturing from AF but struggle how to permanently set up this behaviour on their E-M1. The reason is that the E-M1 like all other Olympus cameras simply offers different AEL/AFL modes for S-AF, C-AF and MF, but doesn't call the relevant ones AF-on. As a consequence, if you search the manual for AF-on or AF on demand, you don't find anything although the feature is available.
With the new features like Live Composite and Digital Shift introduced in FW 2.0 Olympus has now also updated the E-M1 user manuals.