The "Model" field is for entering the main model name or number on the device. This can be a combination of name and number if the item has both - for example, "TR-606 Drumatix".
The "Additional Model ID" field is for the 'fine print' model information that isn't commonly referred to, for example, the E-mu Emax SE Rack has "2020" as it's additional model ID.
note: It is quite common to find a description of the device listed close to the model name/number on the device, for example:
This type of information is really a description of the units function. Please don't add such descriptions to either of the 'Model' fields.