Using Replace/Remove tab you can Replace or Remove a character or combination of characters from selected file names.
So, for example, if a group of files is named A001, A002, A003, etc., you can remove all the "A"s or replace them with "B"s or with your name "John".
If 'All Occurrences' is checked, each replace/remove operation continues until the character/combination of characters (call it a 'String') is no longer found. So if 'All Occurrence' is checked then File Renamer will replace or remove all Occurrence of that string.
if it is unchecked, each search and replace/remove operation will Start on (the) occurrence that you select and continue for the Occurrence count.
You can also wants your search for replace and remove as case sensitive or not. You need to check 'Case Sensitive' check box if you want your search case sensitive.