@Hank21 

Thanks for your reply, but I still don't understand.

Can you explain it a different way?

So which device receiver antenna are we talking about the sensitivity of?

Do you mean the sensitivity of the receiver in the Access Point, or the receiver at the client end, say a mobile phone?

And is that -dBm sensitivity value something we obtain from technical specification for that device?

I thought receiver sensitivity was a fixed antenna design value, and for example a good AP is designed to be capable of better receiving even very weak (wifi) signals transmitted by the mobile phone.

I found the following explanation of receiver sensitivity (aka reception sensitivity) which helped me:-

The intensity of a radio signal follows inverse square law. The further the distance, the weaker the signal.

Transmit power = how strong a signal a device can generate. 20dBm > 10dBm, consequently device generating a 20dBm signal transmits further than a device generating a 10dBm signal.

Reception sensitivity = how weak a signal a device can detect. -90dBm < -70dBm, consequently a device detecting a -90dBm signal can detect signals transmitted from a longer distance than a device detecting -70dBm signal.

https://superuser.com/questions/1674026/how-to-know-from-specs-if-one-router-has-better-range-than-other

I get it now, and in conclusion I understand that:-

Setting the Receiver Sensitivity (RS) slider to the right (Weaker) means that the AP you are setting it for is more capable of receiving "weaker" signals transmitted from a phone that is further away. It increases the range (radius) calculated and shown by the heat map. But ideally you should try to obtain the actual Receiver Sensitivity -dBm figure for that AP model from its technical specs in order to set the slider value properly for the Heat Map tool to show accurate results. If you don't have the RS figures you're only guessing/playing with the heat map which is slightly useful although, to quote a phrase, close but no cigar.

Note that the RS figures given in technical specs resulting from real in-field testing, also depend on other settings such as channel Mhz and bandwidth for which you should take the correct RS figure for the appropriate matching configuration if possible.

The other thing to note about the TP-Link Omada Controller (or website version) heat map tool is that you can't set an RS value for each AP in the network individually. It's not an exact measure and can only give guidance on the ranges to be expected. As you probably know there are many real-world factors such as interference, reflections and so forth that affect the passage of wifi signals.

So it's probably better to err on the side of caution and use the slider to set a stronger RS which makes the heat map calculations based upon the AP only being capable of receiving signal from wireless devices with "stronger" transmitters, i.e. it will display a smaller radius of coverage from the AP.

Receiver Sensitivity is all about the design of the receiving antenna in the access point or router.

A poorly designed lesser capable AP can only hear wireless devices who have a good loud voice - Conversely AP products with receiving antennas of better or larger design are able to make out the whispers amongst all the noise (signal to noise ratio, interference, etc)!

I also found the following articles useful and interesting...

https://community.tp-link.com/en/home/forum/topic/668054