A FasTrak or FasTrak Flex tag will work on any toll facility in California. State law mandates electronic toll collection interoperability, so one tag should work anywhere. I have a Flex tag and have used it on the toll bridges with no problems.
Some history on the FasTrak Flex brand:
Yours truly was the one who came up with the name. I was inspired by what the EZPass group on the East Coast did for their switchable tag for the express lanes on I-495 and I-95 in Virginia, with the special branding of EZPass Flex. The California Toll Operators Committee (CTOC) adopted it after some market research and approval by the Transportation Corridor Agencies (which owns the FasTrak trademark).
The branding came about as a result of an issue with the I-10 and I-110 express lanes in Los Angeles. LA Metro offers only a switchable tag to its customers. It was branded as FasTrak. Yet there are a substantial number of older non-switchable tags in use in Southern California that were issued by TCA and OCTA, also branded as FasTrak. The pricing signs on the 2 express lanes in Los Angeles were displaying a message that HOVs with FasTrak didn't have to pay a toll. However, this was only applicable if you had the switchable tag. No switch - you'd still pay. A few people had caught on to this distinction, and I was concerned that it could lead to legal action because the signs were conveying a misleading message. The toll operators saw this same risk and agreed that a separate brand would help.
Guidance was developed on the use of the brand and one of the things that I made very clear was that the brand should only be used in messaging related to carpooling. For this reason, when you drive the 580 express lanes, most signs only say "FasTrak". The FasTrak Flex brand is only displayed on messages pertaining to HOVs. The messaging on those signs was taken from the I-495 express lanes in Virginia.
The Bay Area was the first region to adopt the FasTrak Flex branding. The express lanes in Los Angeles will adopt it eventually.
As some have noted, a couple of media outlets reported that you could ONLY use the 580 express lanes if you had the FasTrak Flex. That's not true because that would violate our interoperability law. One of the media outlets corrected their report after I brought this to their attention.
One final note: if you use a traditional toll facility like a toll bridge or toll road, the setting on the switch tag doesn't matter. Without getting too technical, the switch setting is tied in with one component of the tag that is typically not scanned by the overhead readers. The tag readers on the LA express lanes and the new express lanes in the Bay Area will scan that component. I'm not sure about the existing express lanes on CA-237 and I-680. Eventually both of those facilities will start requiring all users to carry a tag, though, and require the Flex tag for toll-free travel.