Macros are used in several of the tools and actions within PDF-Tools. 'Macro' is an abbreviation of 'macroinstruction' - a term in computer science for a rule or pattern that specifies how an input sequence of characters should be mapped to a corresponding output sequence. This means that complicated components of the output process can be reduced in the input process, making the work involved both less intensive and less error-prone.
The macros available in PDF-Tools are used when saving files. They are added in square brackets after the file name that the user enters. All macros are detailed below. Unless otherwise specified, macros add the same details as the macro name to the file name:
•Document Info, which has the further options:
•Number of Pages
•Environment Variable, which has multiple options relating to the environment variables of the file.
See here for a technical explanation of macros.