A contract of service is what you, as an employee, are entitled to
A contract for services is what I used to be entitled to as an independent free-lance lecturer
When your employer says “Jump” you maybe ask “How high?” but you will jump (if you wish to keep your job)
When Kaplan, BPP, SWAT, ATC, LSA etc ask if I would make myself available to teach paper X in country Y on specified dates, I had the opportunity to say “Yes” or “No”, dependent upon my mood (and financial situation!)