begin{equation}
label{eq:1}
a^2+b^2=c^2
end{equation}
In eqref{eq:1}, it will be true for right angled triangle, with sides $a,b$ and $c$ is the largest.
begin{equation}
a^2+b^2=c^2tag{2.a}
end{equation}
eqref{eq:2.a} you see that the eqref is not support tagged number added by hand!