Filter on selection will search the active file for all occurrences of the selected string and automatically show all lines containing the string (hiding those that don't).
When a filter is enabled, this command changes to "Clear filter." Clicking this will clear the active filter and show all lines previously hidden by it. You can also press Ctrl + * (asterisk on the numeric keypad) to clear an active filter.
Filter on selection uses the Hide/show selection mechanism to show matching and hide non-matching lines.
See also:
UltraEdit Wiki