You will see MCD_ identifiers for strings and constant strings in the CMarkup method declarations.

MCD_STR is defined in Markup.h as a string class which, depending on your project, can be one of the following:

MCD_CSTR is a constant string argument to a CMarkup method. It is implemented in CMarkup as a struct which allows either a constant reference to a string object (const MCD_STR&) or a zero terminated string.

MCD_PCSZ is a pointer to a constant zero terminated string.

These were introduced with CMarkup release 9.0 in which CMarkup can be compiled for either MFC or STL strings. The MCD_ prefix stands for Markup Compiler Dependency, and is used for all of the defines for the text and string platform differences documented in Unified CMarkup for STL and MFC.