granularity of locks in postgresql

The following lines are from readme file present in the
\src\backend\storage\lmgr folder of postgresql

"If we are setting a table level lock
Both the blockId and tupleId (in an item pointer this is called
the position) are set to invalid, if it is a page level lock the
blockId is valid, while the tupleId is still invalid. Finally if
this is a tuple level lock (we currently never do this) then both
the blockId and tupleId are set to valid specifications."

I dont see any field called tupleId in LockTag..does it have another name?
Also, "(we currently never do this)"--> does this mean that we currently can
acquire tuplelevel(row level) locks in postgresql?
Thank you
Jenny

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