The Selection panel allows you to control the way copy and paste work in the PuTTY window.
By default, when you copy and paste a piece of the PuTTY screen that contains VT100 line and box drawing characters, PuTTY will translate them into the "poor man's" line-drawing characters +, - and |. The checkbox "Don't translate line drawing chars" disables this feature, so line-drawing characters will be pasted as if they were in the normal character set. This will typically mean they come out mostly as q and x, with a scattering of jklmntuvw at the corners. This might be useful if you were trying to recreate the same box layout in another program, for example.
PuTTY's copy and paste mechanism is modelled on the Unix xterm application. The X Window System uses a three-button mouse, and the convention is that the left button selects, the right button extends an existing selection, and the middle button pastes.
Windows typically only has two mouse buttons, so in PuTTY's default configuration, the right button pastes, and the middle button (if you have one) extends a selection.
If you have a three-button mouse and you are already used to the xterm arrangement, you can select it using the "Action of mouse buttons" control.
PuTTY allows the server to send control codes that let it take over the mouse and use it for purposes other than copy and paste. Applications which use this feature include the text-mode web browser links, the Usenet newsreader trn version 4, and the file manager mc (Midnight Commander).
When running one of these applications, pressing the mouse buttons no longer performs copy and paste. If you do need to copy and paste, you can still do so if you hold down Shift while you do your mouse clicks.
However, it is possible in theory for applications to even detect and make use of Shift + mouse clicks. We don't know of any applications that do this, but in case someone ever writes one, unchecking the "Shift overrides application's use of mouse" checkbox will cause Shift + mouse clicks to go to the server as well (so that mouse-driven copy and paste will be completely disabled).
PuTTY will select a word at a time in the terminal window if you double-click to begin the drag. This panel allows you to control precisely what is considered to be a word.
Each character is given a class, which is a small number (typically 0, 1 or 2). PuTTY considers a single word to be any number of adjacent characters in the same class. So by modifying the assignment of characters to classes, you can modify the word-by-word selection behaviour.
In the default configuration, the character classes are:
So, for example, if you assign the @ symbol into character class 2, you will be able to select an e-mail address with just a double click.
In order to adjust these assignments, you start by selecting a group of characters in the list box. Then enter a class number in the edit box below, and press the "Set" button.
This mechanism currently only covers ASCII characters, because it isn't feasible to expand the list to cover the whole of Unicode.