An SDL/GR system specification or MSC document consists of a set of drawings.
The ITU Recommendations do not define any particular mechanism for storing these drawings on computer files. This is therefore tool dependent, and could vary from one (perhaps internally partitioned) file for a complete system to a file for each page of a drawing. SDL has only the concept of separately referenced diagrams (in SDL/GR or definitions in SDL/PR). MSC has a similar concept for referenced MSC diagrams. When the properties of the set of drawings is determined, the references are resolved. How these references are resolved is in some way defined by the tool.
Each diagram has a kind and an identity, and may also have parameters in the heading, and a preamble before the kind and identity. The allowed parameters and preamble depend on the kind of the diagram.
Each SDL diagram can be split into several pages usually numbered 1(3), 2(3) etc. in the Page Heading, but the page identifier before the bracket can be any valid SDL literal name and therefore includes names such as Definitions_01 and string literals such as "Astring_(%^)". The number of pages in the brackets ( ) is always an integer. The kind and identity must appear on every drawing page.
For an MSC document the page heading consists of the vertical page given by and Arabic number (that is 1,2, 3 …,10,11 etc.) followed the horizontal page given by Latin letters (that is A, B, …Z, AA, AB, …AZ, BA … etc.).
The Frame “delimits” the diagram which is useful when the SDL/GR is embedded in another document. However, it should be noted that some items appear outside the Frame. When there is nothing outside or connected to the Frame it can be omitted, but in practice this is never done.
Textual definitions are put in Text Symbols.