As with many things, these are a spectrum. The sub-total clarifies that almost all is resected; the reason being that full resection has worse long-term outcomes, and is not desirable. Partial instils only that it was incomplete, but is used in most instances to mean that only the small affected portion was removed.

Partial: when a part of the meniscus is removed (anterior or posterior horn or middle part of the body or part of the whole length of  the meniscus mainly at the white zone, for example after removal of a bucket handle tear)

Subtotal: when the removed part of the meniscus is near to the red zone and involves the whole length

I agree with the opinion of George. Partial meniscectomy means to remove the damaged part whatever it is. Subtotal meniscetomy leaves in situ the peripheral vascularized zone of the meniscus.

Here is a journal reference that may help you.

The Combined Effect of Frontal Plane Tibiofemoral Knee Angle and Meniscectomy on the Cartilage Contact Stresses and Strains. Annals of Biomedical Engineering, Vol. 37, No. 11, November 2009 ( 2009) pp. 2360–2372

NICHOLAS YANG, HAMID NAYEB-HASHEMI, and PAUL K. CANAVAN