What is a retool?

[oobie38]

Just thinking about this at the weekend, and a quick google hasn't really enlightened me- what is a retool, how does it relate to the original moulds, and what is the difference between that and a new kit- for example are any dimensional inaccuracies able to be corrected.
Are there really cost and time advantages in retooling - I assume that the work is all done in CAD?

[oobie38]

I came across it in some references to Airfix kits- it seemed to me that 're-tool' and 'new tool' were being used interchangeably.