The plague is generally said to have three forms: bubonic, pneumonic, and septaecemic, which can be either primary or secondary. If you first acquire bubonic plague and the infection progresses to your lungs, it is secondary pneumonic (usually acquired via insect). If someone were to be infected by inhaling infected droplets from this person, it is primary pneumonic (usually acquired via human).
The distinction is important because pneumonic plague is invariably fatal without treatment (e.g. in the medieval period) and if treated, it must be done in a very short window to be effective. It is for this reason that most pneumonic plague outbreaks tend to be short-lived as they both spread and then kill individuals 'too quickly' for the disease to progress. In a densely populated urban environment, they can last longer - but bubonic plague, because of reservoirs in rodent and animal populations, will always outlast the pneumonic plague.
Therefore secondary pneumonic exists within a bubonic outbreak that will last longer, whereas primary pneumonic will form its own outbreak that will end more quickly.