I need some help from the smart people in the room.

I recently had a friendly disagreement with a fellow pilot on the minimum climb rate and descent rate while on an IFR flight plan.

From my military training, we were taught 500FPM was the minimum climb or decent rate one could fly while on an IFR flight plan. If you could not meet these minimums, you were to contact ATC.

According to the AIM 2016 4-4-10 Adherence to Clearance d.
-When ATC has not used the term "AT PILOT'S DISCRETION" nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate between 500 and 1,500 FPM until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC.

I can not find the term "optimum" defined in the Pilot Controller Glossary nor the AIM. Anyone want to venture a guess what optimum means? Optimum for me, the pilot? Optimum for the airplane? Optimum for ATC?

Second question, does anyone know any reference to minimum climb and descent rates in ICAO / PANS OPS?

Thanks for the help...