Commenting code in Scheme

The comment character is ; and anything following that on the line will be ignored. The difference is visual. I have often seen a single ; used if the comment is on a line with code, ;; if the comment is on a line by itself, and ;;; if it's a heading of some sort. The most important thing in commenting is likely to follow whatever conventions you see in the code you're working with.

All three of the forms you mention are single-line comments. The double-semicolon may have originally arisen as a cue in Dorai Sitaram's SLaTeX typesetting package that the comment was to be typeset as ordinary text, rather than as program text.

Scheme also has multi-line comments.

In particular, it appears that R6RS, like Racket, allows the use of #| and |# to begin and end multi-line comments. Also, the utterly magnificent #; combination comments out a full s-expression. So, for instance, if you write

#;(define (terrible-function a)
    (totally-broken-code
     here))

The entire definition is considered commented-out.

MIT/GNU Scheme allows the use of ; for single-line comments and #| and |# respectively for opening and closing multiline comments. Just tested on version 10.1.10.