Additional Improvement 的意義

請問一下各位先進

在USPTO對PATENT的前置分類號碼上
有一項
AI Additional Improvement
我在WIKI上看到的是

From 1838 to 1861, inventors could file patent applications on improvements to their inventions. These were published as "additional improvement patents" and were given numbers preceded by "A.I.". About 300 of these patents issued

http://en.wikipedia.org/wiki/Continuing_patent_application

可是我還是不太懂
是說假如發明者做了一些改善之後
對於改善後的專利就用 AI 開頭是嗎???
那對應到現今的情況又是如何呢???
:?