Greetings. I hope this explanation helps.
I suggest understanding this issue is preferable using a vector diagram. The limiting true load limit is 8 kW (horizontal vector) & the limiting apparent power vector is 10kVA. This means that the optimum angle is 37 deg or 0.8, the stated inverter output pf (the resultant limiting reactive vector is therefore 6 kVAr).
Therefore, if your load pf <0.8 the reactive vector limits the continuous output, whereas if pf>0.8 the resistive load vector limits continuous output.