Different garden shed or summer houses manufacturers take different views on the type of floors they put in their buildings. Reputable and knowledgeable manufacturers will ensure the type of material used and the general construction will last a very long time. Some of the cheaper mass marketed garden sheds makers cut all the costs to the bone to make the price as cheap as possible. Nothing overly wrong in that, or for you to buy one of these cheap sheds, IF, you are only looking for your new garden building to last a few years, maybe before you move house.
The shed floor itself would normally consists of floor joists which would sit onto your base and attached to the floor joists would be the floor itself. There are an array of different materials used for this, some of which are, in my view as a garden shed manufacturer of over 33 years, totally suited to outdoor buildings and some which should NEVER be used on outdoor buildings.
The materials which are best to be used are proper tongue and grooved boards made from solid timber. There are also moisture resistant particular boarded floors made to V313 standard which are also 100% suitable for outdoor use. These boarded floors are similar to what you would get in a kitchen or bathroom in a brand new house. With the moisture resistant built in these will withstand everything the elements will throw at it.
The materials NEVER to use in outdoor garden buildings are chipboard, OSB board (sometimes referred to Sterling board or wafer board) and plywood unless is specially made to be used in outdoor conditions. The reason I know this is that when I started my garden shed company back in 1979 I did use chipboard for the shed floors and found quite quickly when it got wet it disintegrated. I moved on from that to OSB board which claimed to be better and again found that when it got wet the damp made the boards swell and eventually fail. So following that experience we made the conscious decision to only use solid timber T&G boards and the special moisture resistant V313 standard boarded floor. Since then we have not had a single problem with any shed or summerhouse floor.
You will find that most shed makers which don't use proper timber for their shed or summerhouse floors will possibly describe their floors as 'sheet material' floors. The reason for this is that they know consumers are getting more clued up as to the failings of chipboard and OSB boards and will reject any sheds with those types of boards. By calling them sheet materials they hope you do not associate their shed floors with these inferior materials. If you are not sure ask the shed manufacturer - are the floors made from chipboard, OSB board, wafer board or sterling board. If yes - do NOT buy unless you accept the risks with those type of boards.
The reason these shed floors are made using these inferior materials is based solely on cost and not on providing a long lasting quality garden shed. You will also find that these cheap floors are excluded from any warranty or guarantee as the manufacturers won't take the risk and leave that up to you. Just ask them the question directly!!
My advice is based on 36 years in the garden sheds business and you can get more information about shed and summer house floors on this page and this page.