Employ your unit in accordance with its capabilities

Successful completion of a task depends upon how well you know your unit's capabilities. If the task assigned is one that your unit has not been trained to do, failure is very likely to result.

Back to the universal principles

Calibration

Under, just right, over.

Situation	Under	Just Right	Over
The ask is outside what the team can do	Accepts it anyway and hopes they figure it out.	Says the team is not trained for that work, names what it would take, or declines it.	Refuses every new kind of work, including a stretch they could make with a short prep.
The load is larger than the roster	Assigns it as if the roster were full.	Cuts the scope, gets more people, or says no before the team breaks.	Protects the team by under-committing work they can already do well.
Unused strength	Leaves capable people on work that does not need them.	Puts unused capacity on a challenge they are prepared to finish.	Fills every idle hour so nobody has slack when the real load arrives.
A harder task would grow them	Keeps them on last month's work because it is safe.	Takes the stretch they are trained to attempt, and prepares the gap first.	Takes a stretch they cannot survive so the team looks ambitious.
A senior asks for work you cannot hold	Says yes and hides the mismatch.	Names the capability you have, the gap, and what you can take instead.	Refuses the slice you can take, so the unit never hears the part it can hold.
You do not know what they can hold	Guesses from last quarter and overloads them.	Checks what they are trained to do before you accept the work.	Runs a survey of capacity so long that the work is already late.

Further reading

Sources and essays on this principle.

Officership Foundations, The Basic School
The published wording of this principle, in numbered doctrine order.
marines.mil
MCWP 6-10 Leading Marines
Doctrine that covers the leadership principles and the traits they rest on.
marines.mil
MCDP 6 Command and Control
Employment has to match what the unit can actually do, not what a plan assumes.
marines.mil