Chlorine and chlorine dioxide are both oxidizers. They try to oxidize everything they come into contact with. However chlorine is different because in addition to oxidation it also chlorinates.
Chlorination involves attaching a chlorine molecule to anything it comes into contact with.
Chlorine dioxide does not chlorinate, so it is "selective."
This has nothing to do with what it oxidizes and destroys.
All chemicals have limited reactions. There are some things that chlorine dioxide will not react with, such as ammonia. Chlorine will chlorinate ammonia and form a THM that is dangerous to the body, but it does get rid of the ammonia. Since chlorine dioxide does not chlorinate, it does not react with ammonia. This makes it "selective."
If you are trying to "purify" the water in a swimming pool, you have to deal with ammonia. However if you are simply trying to "disinfect" the water in a swimming pool, you don't have to worry about the ammonia.
Disinfection is used to eliminate pathogens and ammonia is a disinfectant. Purification is used to eliminate impurities from the water. Both pathogens and ammonia are impurities that need to be removed.
Chlorine dioxide is safer than chlorine for disinfection because the disinfection by products formed when using chlorine dioxide are safer than the chlorination by products formed when using chlorine.
Both chlorine and chlorine dioxide are biocides. Chlorine dioxide is selective in that it doesn't chlorinate. Both chlorine and chlorine dioxide are oxidizers. The main difference has to do with the disinfection by products formed during disinfection.