TDRS spacecraft are geosynchronous communications satellites. They have a large solar array panel to collect power that nominally points toward the apparent position of the Sun. They have smaller antennas used for communicating down to the Earth (and so are nominally Earth-pointing). The main body of the spacecraft is non-pointing.
Solar radiation pressure is a significant force perturbation that must be modeled for the TDRS spacecraft when numerically propagating an orbit. HPOP provides a generic SRP force model that depends on the size of the area of the spacecraft exposed to solar radiation -- a user can specify a constant value (really, an area to mass ratio) in the user interface. While for many applications, this modeling suffices, for high fidelity applications (e.g., orbit determination), the fact that the area exposed to solar radiation pressure changes over time (because of different appendages' pointing requirements) should be considered in the model.
Of course, each spacecraft will have its own set of modeling issues. In many instances, the generic modeling provided by HPOP will suffice for producing the accuracy required. In those cases where the generic models are inadequate, the user can create a HPOP Force Model Plugin to implement the higher fidelity model and have it be used by HPOP during the normal propagation process. Moreover, the user can upgrade the plugin component as better models are developed without having to wait for an upgrade to HPOP.
At first, one may think that writing a software component to achieve higher fidelity modeling is too high a hill to climb. Certainly, the best situation would involve the software already incorporating the higher fidelity model to use. AGI is constantly improving its products by adding more fidelity and capability---but these features are mostly focused on providing generic capabilities that can be used by a wide customer base. Higher fidelity models are by their nature non-generic and applicable to only a few customers.
Generally, the non-generic portion of a model is narrow in scope. The larger portion can simply utilize existing generic models. Certainly, if the price to pay to incorporate a custom model requires reimplementation of all the generic capabilites as well, then you're creating a total custom software solution yourself---one that needs to be continually tested, maintained, validated, and upgraded to meet your changing needs.
The big advantage to using Plugins is the programming code you don't need to write. In contrast to writing all the code yourself, plugins work by implementing only the custom portion of a model and leveraging all of the generic capabilities provided by the plugin point. The only thing that needs to be programmed is the specific custom model itself, the part of the model that is most important to the plugin creator.
Numerical propagation of ephemeris involves many interacting software components: a numerical integrator that solves the ordinary differential equations, a force model that evaluates the environmental forces on the satellite, coordinate frame transformations, ephemeris generation for planetary objects, etc. The force model itself is comprised of several independent constituents: a gravity field, a solid tide model, and ocean tide model, drag, solar radiation pressure, general relativistic effects, etc. These in turn may require further computation of atmospheric density, solar intensity, and other quantities. HPOP provides all of these capabilites (and more) already as a COTS tool.
For the TDRS spacecraft, only the generic SRP model provided by HPOP needs customized modeling. All the other aspects of HPOP's numerical propagation are sufficient for high fidelity. An HPOP Force Model plugin can be created that implements just the SRP computation. In fact, a TDRS SRP model is one of the provided samples. It accounts for the attitude of appendages and even specular and diffuse reflectivity effects.