If you are a sole-proprietor and your business year end is not December 31st, then you would use this form.  If you are like most self employed business owners, your business year is the same as the calendar year and you do not need to worry about this.

https://www.canada.ca/content/dam/cra-arc/migration/cra-arc/E/pbg/tf/t1139/t1139-17e.pdf