A Portable air conditioner, also known as a spot cooler, is a temporary cooling solution when larger a/c systems fail or require maintenance. They are also used in some applications for permanent cooling. This can be smaller server rooms or small equipment rooms.
They are called portable because they come with castors or wheels and can be moved around easily. Portable AC units tend to be smaller than permanent units and so can fit in through standard doorways and entryways. This also limits their capacity.
Portable a/c units should not be confused with mobile a/c units. Mobile a/c units also come with wheels but then to be towed or trucked to a site as they can be much larger. Because they must be set up outside, this limited their portability.
The are many use cases for portable air conditioners. One is for cooling areas when the main air conditioner has broken down or requires repair. These are temporary. The other is for locations where there is no proper infrastructure to place traditional a/c units. There is either no window space or there is no access to the outside for return air. Another is to augment existing cooling temporarily. This includes events like conferences and exhibitions where individual stalls can expect sudden increases in people and additional heat.
Portable a/c units are extremely versatile and with the rapid advancement of technology will find more and more places for its use.