To link to another issue you can copy and paste the URL into your issue and type "i" or "#" with the issue number after it; for example "i23" or "#123" would both create a link to issue 23. When linking to an issue either way Sifter will create a hyperlink with a pound sign and the issue number. For example a link to issue 23 would look like "#23".
When you use the # symbol, you can search for issues by their number or by a single keyword. In both cases, Sifter will provide a list of up to 5 potential matches sorted with the most recently created issues at the top of the list.
Some people may have notifications turned on for when an issue they are following is referenced in another issue that they aren't following. If you reference an issue and someone following the referenced issue has enabled this notification, you'll see their name in the list of who will be notified.