All active PTDAs are addressable, whatever the current process, from a global address in the system arena. However, for the current process an alias address is created using selector 30 and in addition the many of the PTDA field names are declared as public symbols. This allows the fields names in the PTDA for the current process to be referred to directly under the Kernel Debugger and Dump Formatter.
PTDA_Start is the symbol assigned to the beginning of the current PTDA. Using the ? command against this and other PTDA field names allows relative offsets for PTDA fields to be calculated and used in other contexts as offsets from the global PTDA address.
Refer to the following for more detailed information