ThinkPad 600 series
The 600 series was developed as a slim notebook machine line, with the intention of being slightly heavier than a 500 series to include the features of a 700 series equivalent.
This was to create a bridge between the subnotebook and the notebook form factor, in such a way that it has reasonable weight and reasonable performance to cater to those that did not need all-out portability or performance. In other words, it focuses on merging two design goals to create a middle-ground, rather than focusing on one design goal to the extreme.
Following the discontinuation of every numerical series ThinkPads, the 600 series was replaced with the T series as its successor.