When discussing document merging with HTML Master Pages, these key terms will be used:

Resource Document

The document that is actually being requested by a visitor. A resource document should contain one or more SSI-Filler directives and must contain exactly one SSI-Template directive if it is to be merged with a Master Page. Otherwise, the resource document is treated as a flat file and is served up as-is unless it contains PHP code blocks, in which case it is evaluated by PHP before being served up. Should a properly formatted SSI-Template directive be generated by the PHP code, the output is merged according to the directive.

Master Page

The document that is identified in a resource document via an SSI-Template directive as the file to merge with. A Master Page should contain one or more SSI-Container directives. If there are no SSI-Container directives in a Master Page, then it is impossible for that Master Page to accept any BODY content (that is properly wrapped in SSI-Filler directives which precisely match SSI-Container directives in the Master Page) from other resource documents, though the HEAD of both documents will be merged, anyway.

Chained Master Page

When a Master Page also specifies an SSI-Template directive (and presumably, at least one properly matched SSI-Filler directive), it is first merged with the resource document in the current request and then the Master Page becomes an intermediary resource document to the Master Page that it specifies. This creates a "chain" of Master Pages. Apart from the memory limitations of your web server, there is no limit to how "long" these chains may be.

Merge Chain

The end-to-end set of files that are involved in an HTML Master Pages merge request, including the resource document, its Master Page, and all other chained Master Pages, if specified.

Immediate Master Page

While processing Chained Master Pages, the Immediate Master Page is the Master Page that is specified in the "current" resource document. Only the "current" resource document and its Immediate Master Page are evaluated during each stage of the chained merging process.

Subordinate Master Page

While processing Chained Master Pages, the Subordinate Master Page is the "current" resource document which happens to also be a Master Page. This Subordinate Master Page will contain all three of the primary HTML Master Pages directives: an SSI-Template directive which points to the "current" Immediate Master Page, at least one SSI-Filler directive that precisely matches specific SSI-Container directive(s) in the Immediate Master Page, and at least one SSI-Container directive that is wrapped within one of its SSI-Filler directives. This combination of directives in a single file identifies this Subordinate Master Page as a peer in a chain of Master Pages.

Intermediary Master Page

While processing Chained Master Pages, an Intermediary Master Page is the ephemeral merged state of a resource document with its Master Page just before that result is then merged with the next Immediate Master Page in the merge chain.

Final Merged Document

This is the final document that is served up to the visitor after all merging has completed, including fully resolving any Master Page chain.