The multi-field picker is used to select one or more input fields for a property:
If the node has been connected to an output which contains a data set, then fields can be chosen from a drop-down, listing all of the available input fields. If there is no input data, then you can type in the name of fields. It is strongly recommended to connect the node up to provide input data, and then choose fields from the drop-down, in order to avoid mistakes such as misspelling field names.
If you manually type in names, then once you connect up an output pin with data, the fields you've entered will be checked against those which are actually available. Fields which do not exist on the input data set will be shown in red and should be removed.
For each field you can optionally choose whether to reverse the sort order for comparisons, and for string fields you can choose whether to compare case-insensitively or by substring. To see these options, click the menu button to the right of the field:
If you set any of these options, then the context menu button will be shown on a blue background. This menu also allows you to remove fields from the property.
Choosing Compare Substrings will prompt you to specify the start position, and an optional end position, for the substring to use.
The Advanced tab allows you to enter a Python expression to identify fields to use.
If you have already chosen fields using the basic view then you will be able to see the script equivalent for your choices upon switching to the Advanced view.