Memos can be added to the projects as well as time and expense entries. These memos can then show up on the invoices, if required.
Project memos show up in the Memo 2 field of invoices if you have the project rule Use project memo on invoices and the Global Settings rule Show project memo on invoices at the bottom turned on. If only the project rule is active, the project memo is brought forward to the Memo 1 field. If you do not want to use the project memos on invoices, simply remove the rule at the project level. Note that whatever you enter in the Memo 1 and Memo 2 fields shows up on the invoices finally. So you can always overwrite the project memo if it is brought forward here.
You can add memos to an invoice by clicking on that invoice row in the Invoices screen and selecting View Memos. Check CORE Help Center for details.
If you want to have memos per line item on invoices, you can use the Global Settings rule Show time and expense memos on detailed invoices. This rule makes the memos set at the time and expense entry level visible on invoices that show detailed time and expense entries.