This is a user-given string, which is typically used for referencing another command.

The string is any set of characters enclosed in a pair of double-quotes, for example, "My #1 String". The string may not exceed 1023 characters. For example, in
 element_set = "channel"

element_set references the element set "channel" given above. The quotes surrounding a string may be omitted if the string contains only alphanumeric and underscore characters. However, strings that are not quoted are automatically converted to lower case and thus this practice is not recommended.