Each pane of the Text Compare view is a full-featured text editor. You can enter and edit text like any other text editor, although as you do the comparison is continually updated to reflect differences between the panes.
Line mode vs. character mode
The Full Edit 
command toggles the editor between Line Mode and Character Mode. In line mode, editing occurs on a line-by-line basis, so selection, cut, copy, paste, delete, etc. all affect entire lines. While in line mode you can edit individual lines using the Text Details area. Line mode supports disjointed selections. In character mode the main panes support editing text. Selection and editing commands all affect individual characters or spans of characters. Character mode can only have a single, continuous selection.
Visible whitespace
Enabling the Visible Whitespace 
command allows you to see where spaces, tabs, and line endings occur in your files. The following example illustrates each of them.
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Space |
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Tab |
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Windows line ending |
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Mac line ending |
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Unix line ending |
Editing disabled
Beyond Compare disables editing of a file for any of the following reasons:
·The comparison encountered an error.
·The comparison was cancelled.
·The location of the file is read-only (e.g. 7zip/CHM/RAR archives, CD-ROMs, DVD-ROMs).
·The file format conversion doesn't support saving.
·Editing in the parent folder session has been disabled.
·Editing in the file session has been disabled.
·Command line switches have disabled editing.