In certain Business blue prints, i have seen that the project objects like PD, WBS, NWA are described as master data. in some case, the standard templates are specified as PS master data. Of course, as per SAP menu, the work center and statistical key figures are the part of master data.
There is little ambiduity. My understanding is that, master data is the one which is a record for predefined information for repeated usage in transaction. Master data like material, vendor, customer etc falls under this category.
The Project objects are defined specific to transaction.
To avoid various interpretations and arrive unique understanding, i invite comments from experts.