Macro performance attribution investment manager vs allocation effects

What is the difference between investment managers and allocation effects?

My understanding is IM is the value added if you had invested in the investment manager’s allocations that differ from the benchmark. But this should also include “noise” and/or “luck”

I know allocation effects is the reconciling factor. But it sounds too much like, “we messed up our calculation, so let’s plug it”

Is there a good explanation of what allocation effects is?

Investment manageres is the active management.

Alloc effect is the random/luck.

Allocation effects is a plug figure so that the total adds to the total portfolio return; it comprises market timing, cash drag, management fees, transaction costs, whatever.

I call it the whatever return.