haeyun kim said:

Q) I am wondering where does (iii) refer to,

In my house, which is TN-C-S, the green-and-yellow cable from the service head goes to an earthing block, rather than the earth terminal in the consumer unit. The main bonding is connected to this block, an a connection taken to the consumer unit. This is not uncommon. This illustration is from Figure 4.9 of IET Guidance Note 8 Earthing and Bonding, and I've circled in red the separate MET:

haeyun kim said:

and why it is necessary to install the warning notices there?

So no-one disconnects the earthing arrangement, or the connection to the consumer unit, as this may well cause exposed-conductive-parts to have a voltage of 110 V AC from leakage currents in the installation (which can sometimes be fatal). The warning notice can be a durable label applied just above or below the MET, for example as this picture (which is a part of a setup I use to demonstrate a method for testing a consumer's earth electrode):

'Durable' of course depends on the external influences pertaining to the particular installation, so it's not uncommon in commercial/industrial installations to have a more durable plastic, or engraved laminated, notice, perhaps secured with screws.

haeyun kim said:

Q) I am wondering where does (iii) refer to,

In my house, which is TN-C-S, the green-and-yellow cable from the service head goes to an earthing block, rather than the earth terminal in the consumer unit. The main bonding is connected to this block, an a connection taken to the consumer unit. This is not uncommon. This illustration is from Figure 4.9 of IET Guidance Note 8 Earthing and Bonding, and I've circled in red the separate MET:

haeyun kim said:

and why it is necessary to install the warning notices there?

So no-one disconnects the earthing arrangement, or the connection to the consumer unit, as this may well cause exposed-conductive-parts to have a voltage of 110 V AC from leakage currents in the installation (which can sometimes be fatal). The warning notice can be a durable label applied just above or below the MET, for example as this picture (which is a part of a setup I use to demonstrate a method for testing a consumer's earth electrode):

'Durable' of course depends on the external influences pertaining to the particular installation, so it's not uncommon in commercial/industrial installations to have a more durable plastic, or engraved laminated, notice, perhaps secured with screws.