Hot tubs have various filter enclosures, so getting at the actual cartridge filter inside is different on different models, even from the same manufacturer.
Some filter types include the typical skimmer filter that uses a skimmer and then an upright filter canister. These can be either front loaded or top loaded depending on the spa manufacturer and filter enclosure type.
The skim filter type allows for an integrated chemical dispenser to be placed in the center on some upright filter canister types. Having an integrated bromine or chlorine dispenser means the chemicals are added in the circulation line and not from a floater that floats around the top of the water.
One prominent manufacturer of upright filter canisters is Waterway Plastics. Even among filter canister manufacturers, there are many models, sizes and types.
The type of filter cartridge needed for a particular hot tub model depends on what filter canister is installed in that model. So if the filter canister type is exchanged for another one in production, the spa with an identical model name may use a different size filter cartridge. This is why it is important to actually measure the existing cartridge when sourcing replacement cartridges so that you are sure to get the correct filter type.
A vane type filter is also a non-pressurized filter but does not have a front loaded skimmer. This type has a vane that can be unscrewed and then the cartridge is accessed beneath that.
Advantages to this type of filter are that it takes up less room in the spa, so leaving more room for seating. Some clever integrations of the skim filter type also allow for an unobtrusive location of the filter canister.
Another filter enclosure type is the pressure filter. This type of filter actually keeps the hot tub filter under pressure, so it will have a pressure release valve that will need to be activated prior to opening the filter. Then there is usually a lock ring on the outer edge of this filter canister type to be opened and then the pressure lid is unlocked and removed to expose the cartridge filter inside. This type of filter is used for example on the QCA Spas Amethyst model.
Other filter types used for hot tubs include DE filters and sand filters, although these are far less common than the paper cartridge filters.
Whichever filter type your hot tub uses, listen to the hot tub filter song and take heed to the message: keep your hot tub filter clean.
A clean filter not only provides for a great bathing environment, but also saves the equipment.
Have fun in the clean water of your hot tub!