Genie, have you tried a Google search - there are several headings under 73rd Highland Foot Regt.
Looked in my book can't find a definitive answer but I would think that limited service meant that he was signed up for a particular campaign.
Not sure about pensions..the book (Ancestral Trails by Mark D Herber) says 'Soldiers were entitled to pensions for wounds or for completing an agreed term of service' from the 17th c, see if your library has this book. Surviving pension records will be at Kew.
Suey
