%% Operator?

I found the following in the stored procedure, sp_addextendedproperty:

EXEC %%ExtendedPropertySet().AddValue(Name = @name, Value = @value, Level0type = @level0type, Level0name = @level0name, Level1type = @level1type, Level1name = @level1name, Level2type = @level2type, Level2name = @level2name)

Can anyone shed any light on the %% operator? ExtendedPropertySet() does not appear to be a CLR procedure. What is it and what is the %% operator?

Thanks all.