The Search & Replace in Files dialog is used to setup the basic parameters needed by KFileReplace to work. It consists of two tabs, General and Advanced.
When you want to begin a new session the first step is to click on the Customize Search/Replace Session button. Then you must enter the base path and a sequence of shell-like wildcards to use as filter.
Then you could set some useful options, like searching in all the subfolders, doing a case-sensitive search, enabling commands and/or regular expressions, doing a backup copy of each file before replacing.
If you want to start searching, you can put a string in the search box and press , otherwise leave the search box empty and press .
The Advanced tab allows you to set up some useful options to restrict the search to a subset of your target file tree. If you want to run KFileReplace only over files that have a size in the range of 10KB - 100KB, then you could use the size options. There is also a date option that restricts the search in a temporal range, and a last option that allows you to only search for files owned (or not owned) by a particular user (this may be more useful to the system administrators).
 Please note that regular expressions and commands could slow down the speed performances.