Anytime you are curious about something like that, put
i in front of it in the shell:
iex(1)> i '7,9'
This is a list of integers that is printed as a sequence of characters
delimited by single quotes because all the integers in it represent valid
ASCII characters. Conventionally, such lists of integers are referred to
as "charlists" (more precisely, a charlist is a list of Unicode codepoints,
and ASCII is a subset of Unicode).
[55, 44, 57]
IEx.Info, Collectable, Enumerable, Inspect, List.Chars, String.Chars
I.E. a list of integers is a charlist.