There are conventions pertaining to the style a letter is written in. You might have a as an arbitrary element in set A, while a<sub>1</sub>, a<sub>2</sub>, etc., are a sequence of elements in A, while fancy-boldface A or fancy-script A (I don’t know how to render those here) could represent the class of A-like sets. Also, a′ would be another case of a, or maybe the derivative of a, while a″ would be a third case, or maybe the second derivative. Sometimes superscripts or backscripts are used, when subscripts are not enough. Sometimes the Greek equivalent of a Latin letter denotes some relationship between them, e.g. ⍺ is some special version of a.
If the math goes way off into the weeds, the author might even whip out a Hebrew letter or two.
There are conventions pertaining to the style a letter is written in. You might have a as an arbitrary element in set A, while a<sub>1</sub>, a<sub>2</sub>, etc., are a sequence of elements in A, while fancy-boldface A or fancy-script A (I don’t know how to render those here) could represent the class of A-like sets. Also, a′ would be another case of a, or maybe the derivative of a, while a″ would be a third case, or maybe the second derivative. Sometimes superscripts or backscripts are used, when subscripts are not enough. Sometimes the Greek equivalent of a Latin letter denotes some relationship between them, e.g. ⍺ is some special version of a.
If the math goes way off into the weeds, the author might even whip out a Hebrew letter or two.