The weekly tag goes through QA review and we'd recommend using this to prevent problems with your site caused by regressions. But it is in fact just the same as 19_STABLE after QA review (the naming is unfortunate, as we started with 19_STABLE before introducing the weeklies, the weeklies should in fact be more stable). See the orginal forum thread about this here
Security and privacy
Difference between _WEEKLY and _STABLE wrt security
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