The important aspects of reagent purity are the level of purity with respect to compound(s) X, and the purity with respect to compound(s) X required for your particular use case.
For example, if you are conducting a reaction that requires anhydrous reagents but is unaffected by trace metals, then you would want to see a certificate of analysis or equivalent showing an acceptable limit for water in the reagent. Whereas if you require sub-ppm levels of trace metals, then that's the parameter of interest to you and you probably don't care if there is 0.5% water present.
In other words, a 99.5% pure ethanol product and a 99.8% ethanol product could be drastically different in their composition with respect to your needs; i.e. the apparently slightly more pure compound could have orders of magnitude more trace metals present than the slightly lower overall purity product. Furthermore, terminology regarding reagent purity is often similar between different vendors (i.e. pure, ultrapure, milipure, etc.), but frequently has very different implications on the aspect of purity that is important to you.
Although you can frequently make generalizations about the meaning of "ACS Reagent" grade or "[pick-your-prefix]pure" grade, you need to inspect either the specifications on the bottle or certificate of analysis for each reagent to know for certainty that it meets your needs. The simplest reliable source for such information is the manufacturer, most of which have online resources providing this information for their products.