Why do some fonts display AbCdEfGhIj as Preview Text?
Because those fonts are symbol fonts. Initially MainType shows a different preview text (AbCdEfGhIj) for Symbol fonts. You can change this behavior through the Options window (Select Options from the Tools menu and then click the Preview Text tab). At the bottom of this page you can change the preview text used for Symbol fonts. Alternatively you can disable this feature through the “Show this text for Symbol fonts” option.