AQ - Aerobic quotient RE - Running efficiency SEF - Simple efficiency factor
Aerobic quotient is a comparison of running efficiency between the two halves of your run, where a larger number means that you were less efficient in the second half. Positive values indicate aerobic decoupling, with common-thought considering 5% as the upper limit of good training practice.
Simple efficiency factor and Running efficiency are both measures of the efficiency of your run by comparing your heart rate against your speed. SEF is a simple summary metric, calculated as a ratio of your average heart rate during the run against your average velocity. A smaller value means greater efficiency, i.e. less effort for the same speed. RE is a far more involved calculation, that not only examines your run on a second by second basis, but whilst also factoring other parameters such as your resting / standing / and maximum heart rate. It is a range between 0 and 100, with a higher value indicating greater efficiency.
Logic states that over time (with improving 'fitness') SEF should decrease, and RE increase. However, there are many other factors to consider (for example, stage of training, state of recovery and run duration). Often, it is therefore difficult to visually spot such trends.