@Prax1 

Both signal strengths are important.

1. Satellite Deco signal strength tells about placement of Deco unit.

If Satellite Deco signal strength is Weak, Satellite Deco unit is too far from Main Deco. It may occasionally lose connection to Main Deco and that will cause Internet connection interruption for all devices connected to Satellite Deco.

Note that signal strenght may change during the day, so at the beginning check it in Deco app few times a day. If at least once you see it drop to Weak, it means you must move Satellite Deco closer to Main Deco.

2. Connected device signal strength is about quality of WiFi coverage from Deco mesh at your place.

Start with stationary devices that don't move, such as: desktop PC, Smart TV, Smart Thermostat. If any of them show Weak signal, even intermittently, that could mean WiFi coverage is insufficient. That connected device may occasinally lose WiFi connection. Either reposition Satellite Deco or add (buy) more Satellite Deco to improve coverage. Repositioning Satellite Deco could be iterative process - you'll have to check its signal strength after you move it to make sure it is not Weak.

When you are done with stationary devices, meaning they all have Medium or Strong signal, now time to check WiFi coverage everywhere you take mobile connected device such as smartphone or tablet. If you find place where they only get Weak signal, it most likely means you do not have enough Deco units and need to add more.