"At their disposal" = And you can sit in barracks and rot.
"On Call" = And you can sit in the barracks and rot when we want you to.
Both effectively rule out employment by others while they are pay SEMC...and if they want to do that can under either version.
I'd avoid both.
Also, look carefully at what constitutes SEMC. Can the officers/men create a second company for employment to get out of the first ties or are they unable to resign while the contract is in operation?
The other thing to watch is that either may force the SEMC into "And you're contracted to assault the fortress held by the XXth Army Corps and hold it for a year!" because you are contracted to be at our disposal or at our call.