The combination of technologies and frequency bands a phone supports. Modern phones can support over 50 distinct modes.
The modes a phone supports determines whether a phone will work with a specific network, and how well it will work with that network. Only a phone that supports all modes offered by a given network can offer the maximum performance (coverage and data speeds) possible with that network.
Examples of modes include "NR 66", "LTE 12", and "WCDMA 5". There are many other combinations. The first part is the technology, while the number represents the frequency band.
NR (5G) and LTE (4G) are two major current technologies.
Bands used in the U.S. include bands 2, 4, 5, 12, 13, 14, 25, 26, 29, 30, 41, 66, 71, 260, and 261, among others.
It is common for modes to be expressed as a technology followed by a list of band numbers, such as "NR 2/5/66" or "LTE 2,4,5,12".
Band numbers are sometimes written with an "n" or "b" prefix, such as "n66" or "b66", but this is not significant. Band 66 = b66 = n66, for example.
Some of these bands are also known by numbers that represent their frequency in MHz, such as 1900 for band 2, and 850 for band 5. This notation was common with 2G and 3G, but was phased out starting with 3G. 4G and later technologies use the band numbers described above.