PostScript fonts are used for printed titles. If a PostScript is found you should contact editorial for a replacement font so they can test to verify there are no rendering issues with any characters. If a PostScript font is included in an ePub it will render the characters as squares and this can be duplicated in iBooks.
Sometimes you might see a font that was included in source files as .ttc, example HelveticaNeue.ttc. This is a system font and produces problematic issues when trying to insert in an ePub and source files need to go back to editorial for a font replacement.