There is only one file type that uses the .MRK extension:
DPOF Auto Print Order File
An MRK file contains information that describes the order that pictures on a digital camera should be printed when a user prints pictures directly from the camera. It is saved in plain text and is used by digital photo printing services that support the Digital Print Order Format (DPOF). MRK files are located on digital cameras along with the digital image files.
The Print Order file is typically saved in the /MISC folder on digital camera media and is named AUTPRINT.MRK. Images that the MRK file contains print order information for are most likely saved in the /IMAGE folder on the camera.
MRK files are meant to be referenced by the service or printer printing the digital photos on the camera. They are not meant to be manually opened by a user. However, since the files are saved in a text format they can be opened and viewed with a text editor, such as Microsoft Notepad or Apple TextEdit.
Updated: March 23, 2020