[Koha] MARC frameworks - "link"

In the MARC bibliografic frameworks there is the possibility of using a "link" to other MARC fields. According to the Koha Manual there should appear "a little glass" after the subfield. However, I can't find it. I would like to use this link for the following purpose: in the MARC field 440 I do not want to create the authority list, but just simply see if the value I want to insert has been inserted before in another records. In this case, I would like to choose the value I have inserted in previous records. Could anyone tell me, please, how this link function? Thank you a lot Ondrej Mlecka Casa Balthasar Rome