Basically imposter models are low-quality models that are used when you're looking at a unit from a distance. If you don't have them, the unit will be invisible at a distance whereas if you do have them, there'll be a low-quality model when viewing at a distance. Generally I'm pretty sure you want as few as possible, since they'll lag the system so just use them for big things such as skin, helmet, chest, legs and shield.
Code:
<SLOT name="armour" >
<VARIANT_MESH_REFERENCE definition="VariantMeshes/_VariantModels/gondor/clothing/gondor_infantry_armour.VariantMeshDefinition" />
</SLOT>
<SLOT name="arm_armour" >
<VARIANT_MESH model="VariantMeshes/_VariantModels/gondor/arm_armour/gondor_pauldrons.rigid_model_v2" />
</SLOT>
<SLOT name="shield" attach_point="weapon_03" >
<VARIANT_MESH model="VariantMeshes/_VariantModels/gondor/shield/gondor_infantry_shield.rigid_model_v2" imposter_model="VariantMeshes/_VariantModels/gondor/shield/gondor_infantry_shield.rigid_model_v2" />
</SLOT>
So above we have three examples from Rise of Mordor for Attila. The first thing is a variantmeshdefinition, the second thing is a model by itself, and the third thing is a model with an imposter model.
When making a unit, you're making a variantmeshdefinition. So a variantmeshdefinition within that variantmeshdefinition is used if you want to group together a collection of models, and you're planning on using it for quite a few units. Its shown below. Also note how that variantmeshdefinition, within the variantmeshdefinition, also has a third layer of variantmeshdefinitions.
The second thing is the model by itself. Because its just a pauldron, and it isn't a very big model, it doesn't get an imposter model to save space.
The third thing is the shield, which does have an imposter model because its a fairly large piece.
Its generally easier to just apply the imposter model manually, using Notepad++ rather than through the Variant Editor/Assembly Kit. To check how the imposter models look, change the quality button in the bottom left corner from High to Imposter Low as shown here with a vanilla unit which changes its tunic. If you have like 5 different helmet models, use the same imposter for each.