i had the same question (why don't they just make this simple and put the info in the data sheet?)

their answer was similar but slightly different, copied MC's response at bottom here

i think the most important things is to reserve the space so you don't forget and run traces,

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Below is a proposed resolution from Microchip Engineering Support Team for your Ticket xxx
Area : PIC® Microcontrollers
Product Group: S16FHP
Product : PIC16F883
ds wrote >>>
Problem Description:
The data sheet for this part does not explicitly say what to do with the exposed pad on the QFN packages
The exposed pad is not shown in the pin list
I know that by convention these are generally grounded for thermal and / or electrical reasons but the data sheet really should say something
My questions are these:
1] May i leave the exposed pad unconnected ?
2] Is the exposed pad internally connected ?
3] Are there any mandatory mechanical or thermal reasons to connect it when there is no expected power dissipation much above quiescent operating ?
thank you
ds
<<< end ds

Problem Resolution:
Hello Dennis,

Unless otherwise specified in the datasheet, you may connect the exposed pad to ground, or leave it floating, but it may not be connected to any other signals or potentials.
See
http://support2.microchip.com/KBSear...=SQ6UJ9A00I7WF
Regards,
Nicu