Recall:
$\mathbb{R}\,$ is the set of real numbers:
$\mathbb{R} = (-\infty,\infty)$
$\mathbb{Z}\,$ is the set of integers:
$\mathbb{Z} = \{\ldots,-3,-2,-1,0,1,2,3,\ldots\}$
Numbers are usually represented by lowercase letters, like
$\,a\,$, $\,n\,$, or $\,x\,$.
Sets are usually represented by uppercase letters, like
$\,A\,$, $\,B\,$, or $\,S\,$.
A variable with universal set
$\mathbb{R}\,$
(or, any interval of real numbers)
is most likely to be named with a lowercase letter from the end of the alphabet;
particularly $\,t\,$, $\,x\,$, or $\,y\,$.
A variable with universal set
$\mathbb{Z}\,$
(or, any subset of the integers)
is most likely to be named with a lowercase letter near the middle of the alphabet;
particularly $\,i\,$, $\,j\,$, $\,k\,$, $\,m\,$, or $\,n\,$.