The SPropTagArray structure contains an array of property tags.
A property tag is a 32-bit unsigned integer made up an identifier in the high-order 16 bits and a type in the low-order 16 bits.
The identifier is a numeric value within a particular range. MAPI defines ranges for identifiers to describe what the property is used for and who is responsible for maintaining it. In the MAPITAGS.H header file, MAPI defines constraints for each of the property tags that it supports.
The type indicates the format for the property value. In the MAPIDefs.h header file, MAPI defines constants for each of the property types that it supports.