Tis section addresses an important issue related to presolve that the user of MIP control callbacks should be aware of.

Most of the decisions made within MIP relate to the variables of the problem. The heuristic, for example, finds values for all the variables in the problem that produce a feasible solution. Similarly, branching chooses a variable on which to branch. When considering user callbacks, the difficulty that arises is that the user is familiar with the variables in the original problem, while the branch & cut process is performed on the presolved problem. Many of the variables in the original problem may have been modified or removed by presolve.