As found hereThere are actually THREE English-subtitle options for Howl (and the other 2 GHIBLI films released along with it:
1. English Closed Captioning...which includes sound-effects and is more-or-less a translation of the Enlgish audio soundtrack (ie, a word-for-word translation of the the English soundtrack option, not the original Japanese).
2. True English Subtitles. This is more literally a translation of the Japanese language and in many cases differs more noticably from the "English Audio" soundtrack option. Fans and purists should use this option when listening to the original Japanese language to get the most authentic experience.
3. English captions for written signage. This is an unusual option...subtitles only appear sporatically when something in the scene with Japanese wording might have been readable to Japanese audiences (like signs or letters)...and is translated with English subtitles. They only appear a handful of times. Curiously, these subtitles default to "on" and you'll have to cycle through the subtitle menu to shut them off if they become annoying.