Often they are used because many items are not easy to put on a barcode such as alcoholic drinks at a bar, fruit, vegetables, cardboard sheets etc
I do suggest that if you do use PLU or SKU that you stick to the following convention.
PLUs generally, are a 4-5-digit number that are supplied by your supplier. On fruit or vegetables at the supermarket, you have probably seen this code on the little stickers on your items as such they are what we call universal as they can be used by many stores, not just your own. There is a series of numbers between 3170–3269 that you can use if you want for your internal use, but our clients do not need to do this as they can use SKU.
SKU are generally 1-3 digit numbers which you assign yourself. They are as such applicable only to your products and stores. You can customise your SKUs so that you can quickly tell specific information about a product by looking at the number if you want. In my experience, this works very well at the start but after a while gets very messy. So I would recommend you use 1,2,3 etc.
Setting it up is very easy.
1: Open Stock Maintenance and look up the item.
2: Click on the "Prices" tab
3: Scroll to the far right on the pricing grid, and you will see the "PLU" field.
4: Click EDIT and enter a number here (for example, 101 for an SKU and 4152 was the PLU for the apple above)
5: Click SAVE
Now, in the cash register, when you are ringing up a sale enter the PLU into the "price entry" box, and press ENTER and our point of sale will find the item immediately!