When you receive instructions from a client or if you want to remember something important about a job, you can use the Internal notes field at the job level. The data you insert there won't appear in the invoice.
In your list of jobs, the presence of notes will be marked by a little icon next to the name of the job. The latest note is displayed when you hover the mouse over this icon.
If you're outsourcing the job and you would like to add notes and instructions for your supplier, you have to do it using the supplier portal. See this article: https://help.lsp.expert/en/articles/3340222-getting-started-with-the-supplier-portal.