What is the difference between the Brief & Detailed Planetary Ephemeris documents?

Although both documents contain some of the same information, the differences in information content and presentation make each well suited to different tasks.

The Brief document is best for showing changes at a glance in size, magnitude, rise/set/transit times, or motion in the sky over a time period and interval that you specify.  For example, you might calculate an ephemeris for each day in a month to determine which days of the month are best for viewing an inner planet in terms of its size, magnitude and rise or set time.  Another example might be calculating an ephemeris for the moon during a lunar eclipse so that you can see how far it travels across the sky and how that fits with a photographic field of view.

The Detailed document is best to use if you want to do a detailed study of a planet, sun or moon.  The Detailed document can include physical ephemeris information for sun, moon, Mars, Jupiter & Saturn.  (Physical ephemeris data are enabled by default but can be disabled in Report Formatting Options.)

For example, you might want to calculate the latitude & longitude of the center of the sun so that you can estimate the location of a sun spot group or time the rotation of the sun.  The physical ephemeris information shown for the sun includes these data as well as the Carrington rotation number.

Another example might be that you want to know how much the rings of Saturn are inclined to our line of sight on a specific date, and what the angular size of the Cassini division is on that date.  The diameters of the A & B rings are included in the physical ephemeris for Saturn along with the width of Cassini's division and the inclination angle of Saturn to the earth.