After you have specified the groups, listclean needs to know how many passes to perform to clean the data and what matching algorithm to use for each field. It makes informed guesses for these.
For hierarchical data listclean needs to pass once over the data for each level. So for Accounts with Contacts data it requires 2 passes. However, you might want to clean the same level more than once using different algorithms.
listclean also makes informed guesses as to the algorithm to use for each field based on the field name, i.e. the column header in row 1 of the spreadsheet.
You can change the matching rule for each field.
This screenshot shows a list of the types of algorithm which can be used. If you want more detailed control, you can switch to the advanced view.