Funds that have an average credit rating of AAA or AA are categorized as high quality, and those with an average credit rating lower than BBB are classified as low quality. The style box can make it far easier for investors to find appropriate funds. Say you need a fund that carries only slightly more risk than a money market fund. Or perhaps you want a rich income stream but aren't comfortable buying junk bonds. Because bonds typically gain less than stocks over time, their costs become a heavier burden.
Costs are the most important factor when evaluating bond funds, hands down.
Managers with low expense hurdles, in contrast, can offer the same yields and returns without taking on extra risk. Focus on total return, not yield. Yield provides instant gratification in the form of regular income checks. But chasing yield can be costly. Managers will pay more than face value for high-yielding bonds and distribute that entire yield as the bonds depreciate to face value.