Basically, this is a regulation of the technical supervisory authority, which requires all manufacturers of socket strips / adapters to list these instructions. (VDE 0620-1)
1: "Do not plug in one after the other"
Socket strips (multiple sockets) / adapters should not be connected in series. The reason for this is the increasing risk of overloading due to more possible plug-in locations, an additional contact transition resistance and the associated fire hazard, as the strips are only suitable for a certain line withdrawal.
2: "Do not operate covered"
Power strips / adapters should not be operated covered. The reason for this is the danger of heat accumulation and the associated fire hazard.
3: "Voltage-free only when the plug is disconnected".
Safety note, the power strip is only completely voltage-free when the plug is pulled out! Our socket strips with switch switch off the strip 2-pole. This means that when the power plug is plugged in and the switch is turned off, the strip is voltage-free from the switch. Nevertheless, for safety reasons, the power plug should always be disconnected before working on electrical devices.