A job note is for internal information that is specific to that individual job (or estimate) and cannot be seen by your customer. Any information that needs to be remembered/relayed about what will be done on this job or details the customer gave should go here. They will display on the job details page on the app in the field.
These can be entered as you're creating a job, from the 'Line items' page, or from the 'Customer' page. There is a field to the right of the invoice called 'Private notes.' Click in the box and begin typing. The box will expand as you type, so you can enter as much information as needed.
The job notes can be edited or added after a job is created by clicking on the 'private notes' tab along the left side of the job details page.
Click anywhere on the bar to add notes to that specific job.