The spectral mask is generally intended to reduce adjacent-channel interference by limiting excessive radiation at frequencies beyond the necessary bandwidth.
Attenuation of these spurious emissions is usually done with a band-pass filter, tuned to allow through the correct center frequency of the carrier wave, as well as all necessary sidebands.
An FM radio station, for example, must attenuate everything beyond ±75kHz from the center frequency by a few decibels, and anything beyond ±100 kHz (the channel boundary) by much more.
The introduction of in-band on-channel (IBOC) digital radio in the United States has been slowed by issues concerning the subcarriers it uses – and the corresponding increase in the amount of energy in the sidebands – overstepping the bounds of the spectral mask set forth for FM by the NRSC and enforced by the FCC.
Many digital modulation methods such as COFDM use the electromagnetic spectrum very efficiently, allowing for a very tight spectral mask.