Pressing Contents tab in the Toolbar you see a table consisting of alphabet letters having at least one topic beginning with.
For example in the Webster's Unabridged Dictionary it look like this:

Clicking any letter you receive the list of all the topics beginning from this letter.

For example, the list of words, beginning from letter N in this dictionary looks as follows:

Click on topic title will display it.

In addition, in every line of this list there is a sign of small black square (). Clicking the square you can display this topic inside the reference corpus (among all ather topics beginning from the same letters like in real book).

Note.The Alphabetical list of items can be especially useful in case of reading the reference in non-Cyrillic Windows, when the Content panel can be displayed incorrectly and the Alphabetical list is the only navigation tool available.