In the literature, there are many theories and quantities which I usually seen in the context of conformal field theory that described via the notation of  $(a,b);\quad a,b\in\mathbb{N}$  
for example:

1) 6 dimensional (2,0) superconformal field theory (6D (2,0) SCFT)

2) deforming $\text{CFT}_2$ by a (2,2) quasi-primary operator.

3) (NS,NS) sector of superstring theory is given by vertex operator in the (-1,-1) picture

what this notation $(a,b)$ is stand for?
I know that in $\text{CFT}_2$s the Virasoro generators are coming as two copies one for the right sector and the second for the left sector is $a$ representing the number of extended supersymmetries in the left sector while $b$ on the right? if so what are (2,0) represents in the the first example treating 6d $CFT$?