In LTE signaling, Protocol Discriminator is used at NAS layer to communicate the category of messages such as "EPS mobility management messages", "EPS session management messages". These types of message categories contains list of messages such as "attach request" under the category of "EPS mobility management" and similarly "PDN connectivity request" message under the category of "EPS Session management".
This is one way to manage different kinds of messages by using grouping (using Protocol discriminator).
Under each category (EPS Mobility management/EPS Session Management) at max 64 messages can be listed since two left most bit is used to represent the category of messages. 0 1 is for EPS mobility management and 1 1 is for EPS session management and remaining 6 bits will be used for messages under each group.