Most modern smartphones are capable of communicating on different types of communication technologies. These types of technologies can be associated with names such as "2G", "3G" and "4G" or by more specific names such as "UMTS" or "LTE". Each uses a different mode of communication and possibly different radio frequency bands.
This setting allows you to restrict your phone from connecting using certain modes. For example, to allow only "3G" / "UMTS" connections.
It is commonly stated that this can help improve battery life, especially if you limit your phone only to use older, slower technologies. This isn't, however, always the case, for the following reasons.
- The signal strength of your connection has a much higher effect on battery life than the technology used. A strong connection to an LTE tower will usually use less battery than a weak connection to a 3G / UMTS tower. Consider the relative strength and coverage of various network types by your carrier, especially in the locations you're in most.
- Faster connection speeds help your device to spend less time transmitting and receiving information, because any given information transfer will complete faster, returning to a lower-power idle state earlier.
- Newer technologies often have better battery-saving features baked into the technology, though can sometimes be offset somewhat by greater power requirements.
It should be safe to leave this on its default, or most inclusive, setting. However, in special circumstances you can limit it to certain technologies. For example, if you know your current carrier has no 4G / LTE coverage, you can disable 4G / LTE. This may have a negligible saving on battery life, as your phone will, in theory, not need to periodically check for 4G / LTE networks.