Assuming you booked your flight from Philadelphia to Medford and didn't book them as two separate trips... your bags will automatically transfer... if you actually booked the trips separately, you might need to re- check your bag in Seattle. As for how far apart the gates are... that can vary a lot... but SeaTac isn't really that big and it's fairly easy to get from one concourse to another
Not. Sure about the distance but bags should follow you through.
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