Most items you sell will probably be sold as individual pieces, in which case the unit size is 1. This means the basket allows only values divisible by 1 (i.e. integer values).

But for some items, you may want to change this. If you sell cloth for example, it may be that you price it in metres, but can provide it in any length to the nearest centimetre. In this case, you can set the 'unitsize' object config for this item to 0.01 (1cm = 0.01m, the unit you're pricing in). Setting the value to 0.05 would mean only values rounding to 5cm would be accepted, and so on.

It can also work in the other direction. If you sell screws or bolts to the nearest 100 or 50, you can set the unit size to those values, so customers can only add values divisible by 100 or 50 to the basket.

The unit size can be any value, you could set it to 12 if bread rolls are priced individually but only available in 'dozens' or 0.25 if you price an item in kilograms but actually deliver it in multiples of a quarter kilo.

If a user tries to add an incompatible quantity, either from the product page, or by editing quantities in the basket, a warning is displayed indicating that the item can only be ordered in multiples of whatever value you have set.

It is important to remember though that the posted price on the web site is always for 1.000 of an item. So for cloth or rope, it would be the per metre price (even if you can choose fractions of a metre) and for screws or bolts, it would be the single item price, even if the item must be purchased in larger quantities.