The most likely reason for a trademark application to be denied is the likelihood of confusion with other marks. Marks are likely to be denied if there is a similar sounding mark already registered, visually similar, or convey the same meaning. A mark may be granted if the class of goods and services are different from those of the already registered mark.
A mark may be used in commerce, but not registered. In certain geographical areas, these unregistered marks may take precedence over a registered trademark. It is important when searching to do due diligence for these marks.
Under U.S. Trademark law, class headings from the International Schedule of Classes of Goods and Services by themselves are not acceptable for registration purposes. The specific items of goods and/ or services must be listed (Trademark ID Manual)
Design Search Code Manual
Trademark Fee Info
Trademark ID manual
International Classification of Goods and Services