The composite temperature value takes into account multiple thresholds and sensors on the disk, and its purpose is to generate one number which gives you a relative idea of how hot the drive is running. The manner in which this value is computed is implementation specific and may not represent the actual temperature of any physical point in the NVM subsystem. The value of this field may be used to trigger an asynchronous event in the firmware.

This composite temperature value is taken as decisive in terms of warranty claims, so is the most important number.

The values of the other sensors are taken into account when computing the composite temperature. Unfortunately, WD does not make available a circuit diagram of the disk, so we cannot say where sensors 1 and 2 are positioned.

As regarding the temperatures you're seeing, they are well within the operating parameters of the disk.

The WD Black SN770 SSD Review has this to say about the disk's temperatures under load:

If WD’s Dashboard software is reading the sensors in the drive correctly, however, the SN770 does get pretty toasty under a sustained load, at least when Game mode is enabled. We saw a peak temperature of 81°C after two complete writes across the entire capacity of the drive in that configuration. With Game mode disabled though, which is how the drive is configured by default, temperatures are a bit lower. Performing the same test with Game mode disabled resulted in a maximum temperature of 75°C. With temps like this, if you’re going to stick the SN770 in a motherboard that doesn’t have M.2 heatsinks, make sure there’s good air-flow over the drive. Enabling Gaming Mode on the SN770 essentially disables any power-saving / sleep features, so the drive is always at the ready. Toggling the feature on and off had only minimal impact on the benchmarks we ran (as you’ll see a little later), but may theoretically minimize stutter or occasional hiccups in games that don't access the storage subsystem very often.

Typically, an SSD will report such a value and begin throttling above 70C, often around 75C, with a later heavier throttling state before reaching a maximum near 85C.

The composite temperature value takes into account multiple thresholds and sensors on the disk, and its purpose is to generate one number which gives you a relative idea of how hot the drive is running. The manner in which this value is computed is implementation specific and may not represent the actual temperature of any physical point in the NVM subsystem. The value of this field may be used to trigger an asynchronous event in the firmware.

Source : Kingston SMART Attribute Details.

This composite temperature value is taken as decisive in terms of warranty claims, so is the most important number.

The values of the other sensors are taken into account when computing the composite temperature. Unfortunately, WD does not make available a circuit diagram of the disk, so we cannot say where sensors 1 and 2 are positioned.

As regarding the temperatures you're seeing, they are well within the operating parameters of the disk.

The WD Black SN770 SSD Review has this to say about the disk's temperatures under load:

If WD’s Dashboard software is reading the sensors in the drive correctly, however, the SN770 does get pretty toasty under a sustained load, at least when Game mode is enabled. We saw a peak temperature of 81°C after two complete writes across the entire capacity of the drive in that configuration. With Game mode disabled though, which is how the drive is configured by default, temperatures are a bit lower. Performing the same test with Game mode disabled resulted in a maximum temperature of 75°C. With temps like this, if you’re going to stick the SN770 in a motherboard that doesn’t have M.2 heatsinks, make sure there’s good air-flow over the drive. Enabling Gaming Mode on the SN770 essentially disables any power-saving / sleep features, so the drive is always at the ready. Toggling the feature on and off had only minimal impact on the benchmarks we ran (as you’ll see a little later), but may theoretically minimize stutter or occasional hiccups in games that don't access the storage subsystem very often.

Typically, an SSD will report such a value and begin throttling above 70C, often around 75C, with a later heavier throttling state before reaching a maximum near 85C.