In the MPLS network, there are different operation types for labels, among which the meaning of the "pop" action is:

In Multiprotocol Label Switching (MPLS), the "Pop" operation refers to the removal of the top label from a packet's label stack. This action is typically performed by the egress Label Switching Router (LSR) or, more commonly, by the router just before the egress LSR, a process known as Penultimate Hop Popping (PHP). Once the label is removed, the packet is forwarded to its final destination based on its underlying network layer header (e.g., the IP header), as it is no longer within the MPLS domain. This is a fundamental operation for exiting an MPLS network.

A. Replacing the top label with another value is the "Swap" operation, which is performed by transit LSRs along the Label Switched Path (LSP).
B. Adding a label to the label stack is the "Push" operation, typically performed by the ingress LSR when a packet enters the MPLS domain.
C. Replacing the top label with a set of labels is not a standard single operation; it would involve a combination of pop and multiple push actions.

1. Huawei HCIP-Datacom-Advanced Routing & Switching Technology V1.0 Training Material: In the "MPLS Principles" chapter, the section on "Basic MPLS Operations" defines the three fundamental actions. It explicitly states that "Pop" means to remove the label from the packet.

2. IETF RFC 3031, Multiprotocol Label Switching Architecture: Section 3.11, "Label Stack Operations," provides the authoritative definition: "An operation that consists of removing a label stack entry from a packet's label stack is called a 'pop' operation."

3. Stanford University, CS244: Advanced Topics in Networking, Lecture Notes on MPLS: The course materials describe the three basic MPLS forwarding operations, defining "Pop" as the removal of a label at the end of a Label Switched Path (LSP), often at the penultimate router.