((HR zone factor * time in T1) + (HR zone factor * time in T2) + (HR zone factor * time in T3) + (HR zone factor * time in T4) + (HR zone factor * time in T5) + (HR zone factor * time in T6))* Sport Weighting factor = ETS

Endurance Training Score (ETS) is based on the T2 min training score devised by Tony Rice.
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The inspiration was to create a training load score that better reflects the effects of short to medium duration intervals. Due to Heart Rate lag when doing intensity work traditional training load scores like TRIMP can underestimate the actual load of the session.
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ETS is an improvement on the traditional TRIMP score. Not only does it weigh even more towards the higher intensities (top HR zones are multiplied by 9 in ETS and 5.5 in Edwards TRIMP (see below)), it also has a weighting for different sports. It is clear that rowing on the indoor rowing machine is more exhausting than going for a bike ride. In TRIMP this is not taken into account, but the ETS score does account for this.
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Examples of factors for different HR zones and weighting for different sports below:

Example ETS score: 18,617 km row - 1:27:13 duration, ETS - 97.2

Rate of Perceived Exertion (RPE) is a subjective measure of session intensity provided by the athlete. RPE is how hard the athlete feels the session was. RPE is rated as a number on the scale between 1 and 10 with 10 representing maximum effort.

(RPE*Duration = Foster)

Foster et al. (2001) proposed a method of calculating the training load score based on Rating of Perceived Exertion (RPE) on a scale of 1 to 10. This method takes into consideration both the subjective perception of intensity (RPE) and the duration of a training session.

For example: For a session of 30 minutes, with RPE of 5 (hard), the calculation for the Foster score will be 30 × 5 which will result in the Foster score of 150.

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(Average HR *Duration = TRIMP)

TRIMP stands for Training Impulse. TRIMP takes into consideration the intensity (heart rate) and the duration of exercise (in minutes).

TRIMP was originally defined as the product of training volume (in minutes), and training intensity (as average heart rate (bpm)).

For example, 30 minutes at 150 bpm TRIMP = 30 × 150 = 4500.

Ref: https://www.oxfordreference.com/view/10.1093/oi/authority.20110803105725580

OVERVIEW

TRIMP method
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QUICK REFERENCE

A method for quantifying aerobic training load. TRIMP is an abbreviation of TRaining IMPulse. It was originally defined as the product of training volume, measured in minutes, and training intensity, measured as average heart rate (beats per minute or bpm). For example, 50 min at 140 bpm TRIMP = 50 × 140 = 7000. One modified version of TRIMP splits training into zones related to the maximum heart rate (max HR) of the performer: zone 1 is at 50–59% maxHR; zone 2 at 60–69% max HR; zone 3 at 50–79% max HR; zone 4 at 80–90% HR; and zone 5 at 90–100% max HR. The modified TRIMP is calculated as the product of training volume (time in minutes) and training intensity (HR zone). The modified TRIMP for a person with a maximum heart rate of 200 bpm training for 30 min at an average of 150 bpm will be 30 × 3 = 90. In another modification, TRIMP is calculated as a product of the training time in minutes and a Rating of Perceived Exertion where rest = 0, 1 = very, very easy; 2 = easy; 3 = moderate; 4 = somewhat hard; = 5 hard; 7 = very hard and 10 = maximal.
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Take a look at a Ludum blog post for more information about training load scores.