The Auto Append button is only available on the collection toolbar when the ClipHistory tab is active.
By default, whenever you copy some text to the clipboard, AceText will grab that text and put it into a new clip. The clip is added to the top of the ClipHistory. Turn on Auto Append to have the text added to the end of topmost clip in the ClipHistory. This way you can easily create a single clip from many bits of text copied from another application.
When you click the Auto Append button, you will be asked which characters, if any, you want to use to delimit each bit of copied text in the clip. You can select one of the common delimiters, or type in whatever text you want to use as a delimiter. If you do not use a delimiter, the copied pieces of text are appended to the clip one right after the other, without any space between them.
Click the downward pointing arrow next to the Auto Append button to quickly turn on Auto Append using one of the common delimiters.
When Auto Append is enabled, clicking the Auto Append button will disable it. To change the delimiter while Auto Append is on, either turn off Auto Append and turn it on again, or click the downward pointing arrow to access the Auto Append drop-down menu.
See Capture, Enter and Store Text to learn how to use AceText.