When the IRS revised Publication 523 a number of years ago, they messed up the explanation it so it is difficult to understand using the current Publication. But TurboTax will walk you through it (but read the screen very carefully).
It is more complicated than this, but here is a simplified explanation: Let's say you own the home for 10 years, the first 6 years was NOT your Principal Residence, then the last 4 years it WAS your Principal Residence. In that scenario, only 4/10ths of your profit is eligible for the tax-free exclusion. The other 6/10ths would not qualify. But it is a bit more complicated than that
Another thing to consider: In my example above, that home was your Principal Residence for 4 years. That means your US Residence was NOT your Principal Residence for those 4 years, and that will need to be factored in if/when you eventually sell the US Residence.
As I said, the current Publication 523 makes it unclear, but here is a link to an older version that is easier to understand. Look at "Nonqualified Use" on page 15.
https://www.irs.gov/pub/irs-prior/p523--2012.pdf#page=15
