What does DP E and SP mean under Chapter lists in a book For

What does DP, E and SP mean under Chapter lists in a book? For example Imntroduction to Electric Circuits 9th Edition.

Solution

\"SP\" is usually, used to convey that the author/editor is not sure of the spelling of a particular word used.

& I actually did not notice \"DP\" & \"E\" in that book (intro. to electric circuits 9nth edition). If you can specify other books where we can find that it would be helpful.