it's me again. see below:
Although I'm sure that Charlie probably has an opinion on the
subject ! Not that you care what he thinks .
my questions are : what does "not that " mean ?
how do I use it properly ?
I appreciate your help !
This more or less means 'it doesn't involve that ... though'. That is, you mention something (eg. Charlie has an opinion), but you specify afterwards that it doesn't necessarily mean that you care about it.
You can use it at the beginning of sentences, as in your example. This is informal, I believe.