What is the 2281S Dynamic Model Battery Simulator?

The 2281 is a precision measurement, linear power supply. 
That is one of its standard operating modes. 
It can measure low uA and tenths of uA currents with 10nA resolution. 
It can also capture narrow load current bursts as narrow as 140us. 

In the battery simulator mode, the supply uses a programmable model of open circuit voltage and internal resistance as a function of the State of Charge. 
The supply computes Amp-Hours consumed and subtracts that value for the battery capacity (determined by the model). 
The % capacity determines the State-of-Charge which is between 0% and 100%. 
The State-of-Charge determines the battery open circuit voltage, Voc(SoC), and the internal resistance, Rint(SoC). 
The supply monitors current to determine Amp-Hours accumulation to reduce the State-of-Charge. 
The supply also monitors the current to compute the actual output voltage, Vterminals = Voc(SoC) – I x Rint(Soc). 

It is recommended to use an SMU, such as a 2450, to create the model for the battery-to-be-used. 
We have a script which makes the SMU a load to discharge the battery and a model generator to create a model compatible for the 2281. 
https://forum.tek.com/viewtopic.php?f=66&t=140141 
The value of using the SMU is that a model can be created based on the current that the product powered by the battery will draw. 
A very realistic model of the battery (for the specific application) can be created.

There are no specifications for the load, it can be an impedance. Power supplies can power any type of load. 
Load regulation defines how much additional error needs to be added to the voltage output specification because no supply is perfect. 
As the load current increases, there is additional voltage drop in the supply that reduces the output voltage slightly. 

The supply voltage and current output and setting accuracies are specified. 
However, since the instrument is working off a model of a battery whose characteristics can vary from battery to battery, an accuracy specification for the simulator is not meaningful. 
The simulator is the closest we can come to simulating an actual battery.