Assistance is a property of instruction. Intuitively, assistance refers to how much help the learner gets from the instruction as they are learning. The term scaffolding is nearly synonymous.
Operationally, assistance is equated with the probability of a good performance during training. For instance, if the training involves solving a multistep problem, and the experimental instruction causes students to have a higher probability of correctly entering steps than the control instruction, then the experimental instruction has a higher degree of assistance.