Each size is available in 2 types, one with a North pole showing and one with a South pole showing. Both have self adhesive on the reverse side.
If the magnet is sticking to a steel surface, you can use either a North or South pole magnet. They are equally as strong.
If you want to use 2 of these to stick to each, for example to close a cardboard folder, you need to use a North pole and a South pole magnet so that they attract each other (otherwise they will repel).
These magnets are available in small DIY packs and also in Industrial bulk packs for the Printing and Display Industries. All available from stock and available for next day delivery.