According to sec 11 every claims has a priority date. It is this date as on which novelty of your invention is assessed. To make it more simple let me explain you with some examples.
Let us say I filed a
provisional specification on 16th April 2011
complete specification on 16th April 2012
the priority date of claims which are fairly based upon the matter disclosed in the provisional is 16th April 2011.
Let us say I filed two provisionals and one complete specification for both the provisional. This is allowed when both the provisionals are cognate (have related subject matter).
1st provisional on 16th April 2011
2nd provisional on 30th Sep 2011
Single Complete specification on 16th April 2012
Let us say you have 3 claims
Claim 1 is based upon the subject matter disclosed in first provisional. Its priority date will be 16th April 2011.
Claim 2 is based upon the subject matter disclosed in second provisional. Its priority date will be 30th Sep 2011.
Claim 3 is based upon the subject matter disclosed in both the provisionals i.e., claim 3 recites few elements present in first provisional and few others present in second provisional. In this case the priority date of Claim 3 will be 30th Sep 2011.
You may now have a question why the priority date of this claim is 30th Sep, and why not 16th April. The reason is your conception of invention which is claimed in that particular claim, which has few elements from first provisional and few from second provisional, was completed on 30th Sep. You cannot claim priority of 16th April since you were not in possession of those elements as on the date of filing of your first provisional. For example, if I have 50 rupees today with me and I am going to get another 50 rupees after two months, I cannot say today that I have 100 rupees with me. I can only say that after two months when I get the other 50 rupees.
I hope this answers your query.