Line beginning

In general, every new line is a separate checklist item, unless it starts with:

• >> for description, or
• # for the name of the checklist in the Checklist Text custom field, for example: # Default checklist

Making a list

Checklist item line can start directly with item text or Markdown characters like the following: * - + []

Examples

Unchecked items

Following is a list of six valid unchecked items (each with text/summary "checklist item").

As you can see, various prefixes are accepted to indicate the checklist item (e.g. [], *, -, but no prefix works fine too). 

In this simple form, each line is translated into a separate checklist item. 

checklist item
[] checklist item
* checklist item
* [] checklist item
- checklist item
- [] checklist item

Optional and mandatory items

Following is a list of valid optional items (each with text/summary "checked item"). Question mark is used to mark item optional.

? checklist item
[]? checklist item
*? checklist item
* []? checklist item
-? checklist item
- []? checklist item

Following is a list of four items, two of which are mandatory and two others are optional:

[] mandatory checklist item (not completed)
[x] mandatory checklist item (completed)
[]? optional checklist item (not completed)
[x]? optional checklist item (completed)

Checked items

Following is a list of three valid checked items (each with text/summary "checklist item"):

[x] checklist item
* [x] checklist item
- [x] checklist item

Separators (headers)

Following is a list of three valid items and one header at the beginning (headers start with three dashes "—") containing some advanced formatting elements:

--- Acceptance Criteria
- checklist item 1
- checklist item 2 with **bold** text
- checklist item 3 with link to an issue IC-245

Description

Following is a list of two items of which the first contains additional details in description:

[] Unchecked checklist item with description
>> Details for the item above come here. Description text can be long and multiline. It can also contain links, mentions, dates, and other formatting elements. 
[x] Checked item

Statuses

Following is the list of items with different statuses (as defined in your Jira settings):

[] Open item
[open] Another open item
[in progress] Item with status 'in progress' which is still unchecked 
[done] Completed/checked item with status 'done'
[skipped] Completed/checked item with status 'skipped'

Advanced formatting

Items can consist not only of plain text, but also advanced formatting elements like description, markdown, inline code, user mention, due dates, issue keys that will be automatically linked to issues, etc.

Editor

You can examine the text format of the checklist in the editor.