MK, MB, PK and MB refer to the photo black ink cartridges in fine art and photographic inkjet printers. Some printer models will allow you to change these manually; on others the type of black ink is linked to the media setting you choose and is therefore set automatically by the printer.
‘K’ actually stands for key; this comes from the four colour CMYK colour space – Cyan, Magenta, Yellow and Key (black). When referring to MK or PK ink, the ‘M’ stands for Matte Key and the ‘P’ stands for Photo Key. So PK stands for Photo Black and MK stands for Matte Black.
MB and PB are simply alternatives to using MK or PK. ‘B’ simply stands for Black, therefore: PB stands for Photo Black and MB stands for Matte Black.
These abbreviations are most commonly found in file names, such as ICC profiles (where number of characters can be limited). They may also be found within printer descriptions to indicate the colours and number of cartridges of a specific model.
The type of black ink needed to print on an inkjet fine art or photographic paper is usually dictated by the surface finish. For example gloss finishes usually require photo black ink whereas matte finishes usually require matte black ink. If you are unsure of what type of black ink is required for printing, you can always contact the paper supplier to confirm.