What does PBDS St; mean?

#1

After globally writing :-

typedef tree <pair<int, int>, nulltype, less<pair<int, int>>, rb_tree_tag, tree_order_statistics_node_update>PBDS

What does this line mean :-
PBDS St;

And what does the Global Line also means? Every word of it?

#2

its explained in the first video of pbds @Sneha.SNC.007

#3

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