Generally an impeller spacer is used to increase or decrease mixing flow.

Think about a fan. Placed flush against a wall, airflow is low. Use a spacer to move the fan away from the wall and airflow increases.

An impeller spacer will be selected based on impeller speed, viscosity of medium being mixed, and desired mix.

Not sure what you're referring as a spacer.

Follow this http://www.ruhrpumpen.com/files/Brochures/Pump_SCE_B.pdf for the cross-section of an API610 center-line mounted rear pullout single stage vertical split overhung pump (OH2)

You could be referring to the throat bushing or the casing wear ring. API 610 design requires that the pump impeller and rear housing cover can be removed without disturbing the piping and pump suction housing so seal repair or impeller and wear ring replacement can be done without disturbing any piping. That is why spacer couplings are utilized so the impeller/seal/bearing housing assembly can be pulled out from the rear of the pump.

After removing the rear housing, removing the impeller nut allows you to remove the impeller and gain access to the rear casing wear ring. The throat bushing can also be replaced and its purpose is to provide a close clearance opening to the seal chamber. Its removal allows access to the seal and its replacement if necessary. The design all revolves around the ability to perform quick maintenance, maintain pump/piping alignment and provide reliable heavy duty service in the petroleum field.