Spectrum management is the “air traffic control” of the spectrum: planning and assigning frequencies so that friendly systems do not get in each other's way. This is laid down in instructions such as the JCEOI (Joint Communications-Electronics Operating Instructions), which sets out per operation who uses which frequencies, call signs and codes.
Good spectrum management is invisible when it works and painfully noticeable when it fails: without coordination, unintended interference arises between friendly radars, radios and GPS — exactly the effect you want to achieve against the enemy.