Sorry, I know this isn't a great match, but this seemed like the best forum to post this in. I'm looking for a chair mat to protect the carpet in my office from the wheels of an office chair. The suitability of all of the chairmats is described in terms of things like the pile of the carpet (e.g. low, medium, high) and whether or not it has padding. For example, this is the description from one of the mats I looked at: "for commercial and low pile carpets without padding". I don't really know what all of that means. I don't know if the carpet has padding or how high its pile is. I'm looking for a mat to protect the carpet, so I don't want to damage it by using the wrong kind of mat. All I know about the carpet is that it's wall to wall and it's in an apartment. I'd describe it as relatively low / flat. Can anyone offer me some advice on determining the characteristics of the carpet -- what kind of pile it has and whether it has padding -- or other advice on how to choose the right kind of mat?

sorry, wrong message

The reason they are asking if the carpet has a high or low pile (Pile = how thick the yarn is from where the yarn is fastened to the carpet backing) is because on a good grade of chair mat, the underside of the mat has a grid of protrusions (like rows of small needles about 1 to 1 1/2 inches left to right and north to south).
That prevents the mat from crushing the carpet it is resting on. These "needles" or "pins" are usually made of the same material as the mat (hard, heavy-wear plastic). On a carpet that had a very high pile (1/2" to 3/4" or more) plus padding (of 3/8" to 1/2" thick), you would want to order a chair mat with a grid of longer protrusions. This would keep the chair mat from crushing the carpet completely flat...bad news if it was crushed completely flat and you decided to rearrange your furniture.
See if your description gives you more details, like how low or high the grid of protrusions are on the underside of the mat. The protrusions have two purposes (1) to protect the carpet from being crushed (2) to prevent the mat from slipping. A very low pile, commercial style carpets without padding, you would choose the shortest protrusions possible.
I think they call the protrusions "cleats", but I'm not sure.