The system supports the use of barcodes so that you can use a barcode scanner to quickly and accurately enter product identification.
That is, instead of entering 'Advantix X Large Dog 3 Pack (25Kg+)' (probably by keying 'adv' then pressing Enter and then selecting from the long list of Advantix products), you can use the scanner to enter its barcode, and this will be used to find the product.
To get this to work, you need to enter the barcodes as product identities for each of the products that you want to be able to scan, and the scanner will be really useful when you enter this data as it saves keying the 12 digit number.
What is actually happening is as follows: if you enter a product (say on an invoice) as a number, then the system does an identity search of the products for that number, and if it finds only one, then it uses that product, otherwise it displays the select screen.
Note that there is no reason why you should not 'barcode' your own products and services. There are free programs available that will generate the required graphics, and you could print up a page listing these and keep it near the work station so that scanning calls up 'Consultation (9pm-Midnight)'.
A final warning: if you do use your own barcodes, make them long. As explained in identities, the IDs are also searched, so a barcode of 123 will find any IDs starting 123.