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Because you don't want the appendix titles to appear in the text, you should not use captions for them. This is what TC fields are intended for: text that appears in a table of contents (or in this case a list of appendices, which will be just a variation of a table of contents) but not in the text.
First, click the ¶ button on the Home ribbon to display nonprinting and hidden characters. That's because a TC field is automatically formatted as hidden, and you need to be able to see its code to edit it.
With the cursor at the beginning of Appendix A, choose to insert a TC field, using the Insert > Quick Parts > Field dialog:
In the Text Entry box, enter the text you want in the list of appendices, but leave two spaces for the appendix letter. Then click the Field Codes button at the bottom. This shows the advanced field properties. Click the Options button at the bottom to open the Field Options dialog.
With the \f switch selected, click the Add to Field button. Then type a space and the letter X at the end of the field code. (Any letter will do.) Click the OK buttons in both dialogs. The field code will appear in the document at the cursor location.
Next, click between the two spaces following the word Appendix in the field code. Again use Insert > Quick Parts > Field, and this time insert a Seq field. At the end of the field code, type the letter A and a space. (Again, any letter will do.) Click the Options button. On the General Switches page of the Options dialog, click the A, B, C formatting and click Add to Field.
Click OK in each dialog. The field in the document should now look like this:
The letter A here is the result of the Seq field.
Copy and paste this field to the other appendices, and edit the field code to change each one to the correct title.
To create the list of appendices, go to References > Table of Contents > Custom Table of Contents. Clear the Styles check box and the Outline Levels check box, and check the Table Entry Fields check box, then click OK in each dialog.
The field that this inserts will show an error message "No table contents entries found" because the field code is not yet complete. Right-click the error message and click Toggle Field Codes. After the \f switch in the code, type a space and the letter X (the same one you added to the \f switch in the TC fields). The code should now look like this;
Click on the code and press the F9 key to update the field. The list should now be correct.
If you add, delete, or move any appendices afterward, select the entire set of appendices and press F9 to update all the fields. The Seq fields will adjust to the new arrangements. Then click on the list of appendices and press F9 to update that.