The jamb width is the width of the jamb as it meets the wall stud of your house.
Many people measure the inside jamb of the door and decide that this is the jamb width; this is not always the case.
In order for the door to fit accurately to the opening without the possibility of future problems, you have to get the true measurement of the wall for the jamb.
Interior and exterior walls have different measurements due to different types of building materials present.
Keep in mind that a traditional 2"x4" is actually only 1-3/4"x3-1/2".
For instance, an interior wall is framed with 2"x4" studs mainly with a 1/2" sheet rock on either side.
The exterior of the house may be framed with a 2"x4" also, but will have brick, stucco or siding.
It is best to remove the casing and brickmould to see between the stud and the jamb for an accurate measurement.
Depending on the situation, you will need a 4-9/16", 5-1/4" or 6-5/8". There are some special occurrences where you may need a 7-1/2" jamb, but you will always need to check your wall prior to purchasing any jamb materials to avoid a headache.
It is always best to have a professional have a look. Find a local contractor in your area if you have any questions about your house.