You can create as many canned comments as you'd like by adding non-inventory items with descriptions. Example, C1, C2, etc. with associated descriptions.
You can also cursor to the description on any item entered in POS and change the description on-the-fly.
To add a comment to an item in POS, cursor to the ILC (Item Lookup Code) and press Ctrl-F10 (line item comment).
To add an invoice comment use Shift-F9 Details (reference and comment fields).
You can also add notes to selected items (Item Properties | Special | Notes) and choose to have Notes included with item descriptions in POS (File Configuration | Options | Item).
To create canned warning messages or special instructions that do NOT print on receipts, use Manager | Database | Item Messages. Apply the canned message to an item under Items | Properties | Special. This will pop an alert to the cashier whenever the item is entered in POS.