If you have "stable" then your system will upgrade from Jessie to Stretch when Stretch becomes the new stable (and then from Stretch to Buster, and so on).
If you have "jessie" then your system will stay on Jessie as it moves from Stable to Oldstable, and then eventually become obsolete when Jessie hits end-of-life.
What you want to watch out for is to avoid mixing "stable" and "jessie" because that can cause big problems when Stretch is released.
I personally would use "jessie" not "stable" so that, if I decide to upgrade to Stretch, I can do so manually at a time and place of my choosing.
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