Posting is decided according to their business requirements.
There are several departments within Infosys on which your posting depends.
There is a Corporate Planning Group (CPG)at Infosys, Bangalore which makes all the important decisions.
All the senior HR executives and Directors belong to this group alongwith the lovely lady who came to conduct your campus interview.

(I checked about the lady who came to conduct our interview and found it to be like that).
There are various Business Units in Infosys like
1. Communications, Media and Entertainment (CME)
2. Healthcare
3. Retail
4. Banking and Capital Markets
5. Business Intelligence etc.
This is domain classification. Another classification would be like
1. IVS : Independent Validation Group
2. SI: Systems Integration
3. IMS: Information Management Systems.
These units would tell their requirements to the CPG according to the projects they have in hand.
If CME unit at Pune will say we need 100 Java trained people the CPG will give postings to 100 Java trainees as CME, Pune.
Some other unit will say we need 200 .NET people at Trivandrum and the CPG will do so.
This is the reason that even if you are allowed to swap, you still cant change your business unit and cant swap with a person from another stream because they have a specific requirement at the location and business unit which needs to be met.
The posting is usually announced in the last month of training, mostly in the last 2 weeks. After you have been assigned to a business unit by CPG, the head (or HR or whoever) of that business unit will contact you and all your communication will be done with him only and you dont directly need to interact with CPG.
As you can see, till now, i have not even mentioned HRs at Mysore. So now you know why you cant request for a location of your preference because they simply dont have any say in this. Their only role is to forward your swap request to CPG or The Business Unit you are posted at. The responsible people for your posting are the ones in CPG and you wont ever interact with them. This is one of those very smart processes that they have in place.
You will get enough time for swapping as it is announced 2 weeks before. And as soon as you get your posting, hundreds of mails from unknown people will overload your 10 MB inbox asking if anyone wants to swap. But be careful, sending such mass mails (and posting about swap on their online bulletin board) is against their policies and they have the right to cancel your swap if you found a swap through these methods.
So how to swap? There is a Swap Portal on their intranet website which you can access only after you have got your posting. You have to submit your request there with details like employee number, unit posted to, current location, preferred location etc. As soon as there is a match for you, your locatin will be swapped with that person.
It can also be done if two friends walk upto the HR and submit their request. You will make friends from Chennai etc who would be wanting Chennai and got Pune and you want Pune but got Chennai. In such a case, you two can go to the HR and say we want to swap our locations. Again, as i said before, your business unit and stream training needs to be the same.
This deserves to be a post on my blog...
