As stated above, the komejirushi or Reference Mark (※, Unicode 203B) is generally used preceding an additional comment or supplemental information. The best way to translate it into English is to write Note: (including the colon) followed by a translation of the comment text. Other possibilities include Attention or Important.
I have seen translations leave the komejirushi as-is, but it is not a symbol with a generally recognized meaning in English. Others "translate" it as an asterisk, as if the two symbols are interchangeable, but that is true only when it is used as a footnote marker, which is rare. See below.
The name "Reference Mark" suggests the symbol could also be translated "Reference," often abbreviated "Ref.," or "For Reference (Only)." Although widely used by Japanese, these terms do not usually fit the actual meaning well. Another possible translation is "Reference information," also a bit off-target in most cases, besides being wordy and stilted.
One might consider "Refer to (...)" if there is a citation of some document or page number, but that is also almost never the way it is used. (The Japanese term for Reference is 参照 sanshou and when there is a source listed it can simply be translated "See" or "Source.")
The komejirushi is also used to preface a proviso, condition, or an exception to a rule. In this case as well, the translation Note(:) can serve. Other possibilities are Caution, Warning, or Exception, followed by a colon and the comment.
The abbreviation N.B. for the Latin nota bene ("note well") may also be used, but many people are unfamiliar with this somewhat bookish term. In many cases, it would be fine to simply use the word However followed by a comma and the comment.
There are the rare cases where the komejirushi is actually inserted to mark a footnote. If used in isolation and in a matched pair (with the symbol inserted both immediately following the text and preceding the footnote), an asterisk (*) is the correct translation. But in some cases the komejirushi is used for multiple footnotes and accompanied by a number (Arabic numeral), such as "※１", etc. For one or two footnotes on a page, an asterisk and then a double-asterisk (**) with no numerals is the preferred solution.
Finally, there is an old custom of using the dagger (†) and then a double dagger (‡) for the third and fourth footnotes when there are not more than four in all. A more modern method may be to use only superscript numbers with no asterisks or daggers. (However, if the footnoted items are actually numerical data, it is preferable to use either asterisks and daggers or alphabetical superscripts.)