This is a powerful tool which allows you to have one type of text spoken before all other items. For example, you could have lyrics spoken first so that you can quickly learn the words to a song.
The dialog only contains one list and an OK button. Use the arrow keys to choose which style of text you want to filter and then press the OK button. If you do not want to filter any text, just choose "none" in the list.
Once you have chosen a filtered text style, any text which matches that text style will be spoken before any other items, een if the text comes after the start of the selected note.
The most important aspect of filtered text is that it is spoken differently. Normally, Sibelius Access simply speaks the text as it appears and makes no attempt to "pronounce" the text properly. For example, a chord simbol of Bbm7 will be spoken as "Bbm7". However, if you have chosen chord symbols as the filtered text style, it would be spoken as "B flat minor 7".
Filtered text items are also nearby items. This is just an elaborate way of saying that Sibelius Access begins by gathering all the nearby items, and then filters out any text items which match the filtered text style.
There are several other keystrokes which make use of filtered text.