Official names are what I call names created by government agencies. Most of those used in the U.S. are a hybrid between common names and scientific names, often the scientific names that were used in the first part of the twentieth century. Taxonomy refers to forming groups. So remember, taxonomy first . Common names .
Indian ricegrass is an example of a common name.
Bluebell' is an example of a common name that means different things to different people. All the species called bluebells have blue, bell-shaped flowers but they are not particularly closely related. Bluebells in England are monocots, what North American horticulturalists call wild hyacinths, but in Scotland and western North America they are dicots, but not the same dicots in the two countries. Pepper is another common name with multiple meanings. Ask where the peppers are in a local supermarket and you will be sent to the produce section where you will find bell peppers.