The Test Chamber Sign is a tool used in Portal 2 to tell the player what elements will be presented in the level, and the progress they have made out of the total amount of levels.
Creating the Sign
1) Place a func_instance on the wall where you wish the sign to be.
2) Choose the VMF filename "instances/signs/info_sign_sp_clean.vmf".
3) Name the instance "level_sign".
4) Place a trigger-texture before the player will run up to the sign and tie it to a trigger_once. Make the output for the trigger_once:
By default, this instance creates a sign that displays "00" and "None" for level number and level progress, with no hazard icons. Test sign data appears to be loaded based on the level name, from the file "scripts/vscripts/transitions/sp_lightboard_icons.txt".
If you make your own sp_lightboard_icons.txt, edited to include your own maps and place it within a VPK package (still following the same file structure), your signs will work. The official single player maps can be removed from your EDITED COPY, but make sure to leave the original in "scripts/vscripts/transitions/sp_lightboard_icons.txt" UNEDITED. The formatting of the sp_lightboard_icons.txt is explained within it, with comments. This method will not change the original file, but the game will use your version while running your map from the VPK.
Although it does seem that if you have a custom addon VPK with a custom sp_lightboard_icons.txt within it, the official single player signs and any in other VPKs, won't work. There can only be one available for use at one time, although a single custom sp_lightboard_icons.txt will override the main single player one.