I was studying the designation of names to refrigerants

I got the following basic formula  :

R - (m - 1) ( n + 1) ( o )

where m = number of carbon atoms in the refrigerant n = number of hydrogen atoms in the refrigerant o = number of fluorine atoms in the refrigerant

so, R - 134a has:

1 + 1 = 2 carbon atoms

3 - 1 = 2 hydrogen atoms

4 fluorine atoms

now, what is 'a'?

I was studying the designation of names to refrigerants.

The following is the basic formula:

R - (m - 1) ( n + 1) ( o )

where:
m = number of carbon atoms in the refrigerant 
n = number of hydrogen atoms in the refrigerant 
o = number of fluorine atoms in the refrigerant

So R-134a has:

1 + 1 = 2 carbon atoms
3 - 1 = 2 hydrogen atoms
4 fluorine atoms

What does the 'a' at the end mean?