RF mask (power spectrum measurement bandwidth)
The simple answer: when you take a power measurement using some piece of test equipment (such as a spectrum analyzer or an RF power meter), the reported value indicates the amount of power measured within some detection bandwidth of the instrument. For some devices, that might be a fixed value, determined by the filtering present in its RF front end. Other types of equipment, like a spectrum analyzer, will typically have adjustable bandwidth (which in this case would usually be called the instrument's resolution bandwidth).

To see how the bandwidth setting affects the measurement, try injecting a modulated signal into a spectrum analyzer and see how the power measurements vary as you change the resolution bandwidth: you can get very different answers. Take a common $\frac{\sin{x}}{x}$ spectral envelope as an example: 


If you use a very small measurement bandwidth, then you're taking power measurements over very small swaths of frequency. This allows you to observe the detail in the spectrum's shape. 
As you increase the resolution bandwidth, your instrument will take more and more of the signal's spectrum into account for the power measurement, and the reported values will become more of an overall measure of the total received power.


As was indicated in the comments already, the specification calls out a specific bandwith to ensure repeatability. Specifications are used to validate implementations as conforming to an existing standard. This particular scheme for measuring the emission mask of a transmitter implementation is likely in use because that was the one that was chosen when the standard was designed. Due to the imprecise definitions of concepts like SNR and bandwidth, there are many ways that you could define such a requirement, but this is the one that the authors chose. That doesn't make it right or wrong; it just is.