This field specifies what types of data connection should use this APN configuration. This is the second most important setting after the APN field itself.
If your carrier allows all data (including MMS, if you use it) to use the same APN configuration, then this field may contain the special value of "*" (an asterisk; the actual field should not contain the quotation marks). This explicitly tells your device to use the same APN configuration for all types of data.
Otherwise, you may direct different types of data to different APN configurations by specifying differing values in this field for each APN configuration. Note that you can specify multiple values in a single field, by separating them with commas.
As usual, you should follow your carrier's recommendations for what to include in this field for each APN setting.
The APN configuration you want to use for general internet data should have "default" in this field. This value indicates that your device should use this APN configuration by default for all types of data communication, unless specifically overridden elsewhere.
When using the value "default", it is not required to add additional values. For example, "default,supl" will be functionally the same as using just "default", assuming "supl" is not explicitly specified anywhere.
The APN configuration you want to use for MMS (Multimedia Messaging System) should have "mms" in this field. This indicates that your device should use this APN configuration for MMS communication.
MMS communication requires additional fields to also be configured, such as the MMSC field and in some cases the MMS Proxy / MMS Port fields.
Indicates the current APN configuration may be used for Secure User Plane Location (SUPL) data, a feature that allows your phone to determine its geographic location from the carrier.
Normally, SUPL data will be able to use the same APN configuration as general internet data, so there will be no need to specify this value explicitly: it will be covered by "default" and will still work fine.
Less common values
Indicates the current APN configuration may be used for Dial-Up Networking connections. This refers to an outdated method for using your phone to emulate a dial-up modem and is not the way that modern smartphones do tethering anymore. There should be no need to use this value.
Indicates the current APN configuration should be used when applications request a HIPRI connection. This value never has any effect in a normal setup.
When making a connection, Android applications may specify a connection type of "HIPRI", which affects the way Android itself routes the data. Normally, HIPRI data will just use the same APN configuration as general internet data, and there will be no need to specify this value explicitly. Specifying this value would only have an effect if you place it in an APN configuration other than the one marked "default", and carriers are unlikely to ever require such a thing.
Many websites falsely claim that adding "hipri" to an APN type field will boost performance or enable a new feature. This is not the case. It's important to understand that adding an extra value to this field does not have the ability to change performance or turn features on or off; the field merely tells Android which APN configuration to use for different types of communication. The absence of a "hipri" value still allows HIPRI communication via the APN configuration marked as "default".