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Whenever the supply to a Transister (or TTL circuit) is connected between the collector and ground it is a standard to notify it as Vcc.
If you connect the supply between Base and ground we will call it Vbb
and the same goes with Vee for emitter to ground. Since there is no notion to identify ground (0) this method was simply adopted.
This method is a standard specified by IEEE.
This information was obtained by me from the book 'Digital Electronics' by Malvino and Leech.