This is commonly used for "Married Filing a Joint" return. To use this filing status the taxpayers must be legally married on the last day of the tax year. It is restricted to a man and a woman as husband and wife, but otherwise follows state law in terms of common-law marriages, etc.
MFjob files are plain ASCII files that contain instructions for MFjob, an emTeX program that builds groups of MetaFont fonts. MFJ files can be created by hand to automate the process of building a set of fonts. They are also created by the emTeX DVI drivers if automatic font generation is being used.