Bonding in a soldering station
What is the need for this?


To expand on the comment from PlasmaHH, that soldering station (and several other Weller products) give you options about the earthing of the soldering iron tip.

There is a specific socket on the soldering station, where that jack plug (and any connections to it - see below) is inserted.


  I never saw this in previous stations.


I have seen several soldering stations that provide a separate terminal for connecting the iron tip to a local ground. This use of a jack plug just offers more options (explained below).

Some soldering stations from another brand only use the mains earth connection to ground the tip (no other options). They monitor that mains earth connection and shut down the soldering station of that mains earth connection is broken.

There are probably other approaches by different manufacturers too.


  Do I really have to earth this? 


You usually don't want the tip of a soldering iron to be "floating".

By default, using that Weller 81i soldering station, the soldering iron tip is connected to the earth pin in your mains cable:



That point in the manual is saying, without the jack plug inserted, the soldering iron tip is "hard grounded" i.e. it is directly connected to the soldering station's mains earth connection.

Therefore the question you need to ask yourself is: Is the earth pin in your mains power socket, connected to a low impedance local earth? If so, then the soldering station will behave like typical mains-powered irons, with the tip connected to the mains earth, when that 3.5mm plug is not inserted (i.e. the "as delivered" state).


  Which one is safe in my case? 


You have not explained "your case" and what exactly you need. However there are only a few choices. Here is a slightly different version than the one you kindly provided, of the relevant extract from the manual:





Going down the list in that image and using the same reference letters:


(a) Hard-grounded

This is the "as delivered" state, with the jack plug not inserted.

The soldering iron tip is directly connected to the earth pin of the mains input socket of the soldering station. If you want the tip to be earthed, then you are responsible for ensuring that the mains earth connection to the soldering station is good enough and suitable for that purpose. Some examples of things which could go wrong, would be if you have a disconnected mains earth connection, or where your mains earth and neutral are swapped.
(b) Equipotential bonding 

This is when you connect a wire to the centre contact of the jack plug.

Now the soldering iron tip is connected only to whatever is at the other end of that wire, and not to mains earth. This connection via the wire could be a local earth, to perform equipotential bonding between multiple parts of an ESD setup, where the mains earth connection is not suitable for any reason.
(c) Floating

This is when the jack plug is inserted, but with absolutely no connections to it.

Again, the soldering iron tip is connected to the centre pin of the jack plug (like in option (b) above) but in this case, there is absolutely nothing connected there, so the tip is floating. This would usually be banned by an ESD policy. At the moment I can only think of one, unusual, use for doing this.
(d) Soft-grounded

This is when the jack plug is inserted, with a resistor (e.g. 1M&Omega; shown in the picture) between its centre and outer pins.

This is a variation of option (a) - the soldering iron tip is connected to the earth pin of the mains input socket of the soldering station via that resistor. Again this relies on the mains earth connection to the soldering station being "good enough".

Using "soft-grounding" (and choosing the value of that resistor) is a compromise between keeping the soldering iron tip firmly at the same voltage (i.e. ground) as other parts on your ESD bench (needs low resistance to ground), and reducing possible damage to a charged device, by it discharging via the soldering iron tip (needs higher resistance to ground).



  I couldn't find any information in the manual.


This is probably because the option you select depends on your company's local ESD policy, which the soldering station manufacturer doesn't decide. The default option (soldering iron tip "hard connected" to mains earth input) is a common standard - but not the only one. That is why having choices can be useful. If you have questions, check the ESD policy of your organisation regarding soldering iron tip earthing.

In an "amateur" situation where your ESD procedures may be incomplete, leading to some devices remaining charged before soldering, then I suggest the "soft-grounding" option with a 1M&Omega; resistor (with a confirmed "good" earth connection to the soldering station) should be considered.