JPL planetary ephemerides are generally created to support spacecraft missions to the planets. Selected ephemerides are recommended for more general use.
The latest JPL ephemeris with fully consistent treatment of planetary and lunar laser ranging data is DE440 (Park et al., 2021). The dynamical model for DE440 includes a frictional damping between the fluid core and the elastic mantle. This damping term is not suitable for extrapolation more than several centuries into the past. In order to cover a longer time span, the ephemeris DE441 was integrated without the lunar core/mantle damping term. The positions of the planets for DE441 agree with the positions on DE440 to within one meter over the time covered by DE440. For the Moon DE441 differs from DE440 mainly in the estimated tidal damping term causing a difference in along-track position of the Moon of ~10 meters 100 years from the present and growing quadratically for times more than 100 years from present.
The JPL planetary ephemerides are saved as files of Chebyshev polynomials fit to the Cartesian positions and velocities of the planets, Sun, and Moon, typically in 32-day intervals. The positions are integrated in astronomical units (au), but with polynomials stored in units of kilometers. The integration time units are days of barycentric dynamical time (TDB). In DE440, similar to DE430, the astronomical units have been fixed to the value 149597870.700 km as adopted by the International Astronomical Union in 2012.
In the Horizons System, the result is based on TDB time, and you input TDB time rather than UTC. So, if you want to compare your results with the Horizons System, you need to send UTC time to the JPL_Eph_DE441 function, which corresponds to TDB input for the Horizons System. In the vector table settings of the Horizons System, for the reference plane, choose "x-y axes of reference frame" and choose geometric states for vector correction. Please remember to use TDB time for your computations when your aim is not the comparison of results with the Horizons System.
If you have any problems using the Horizons System or my source code, please send me an email (email@example.com).
Park et al., The JPL Planetary and Lunar Ephemerides DE440 and DE441, 2021.