- BIP 9 ("version bits") is a standard for proposing Bitcoin upgrades or "deployments". Code for this is included in Bitcoin Core.
- BIP 141, together with BIP 143 and BIP 147, ("SegWit") is a deployment which follows the BIP 9 standard. Code for this is included in Bitcoin Core.
- BIP 91 ("SegSignal") is a deployment which uses the BIP 9 machinery while not strictly following the standard. Code for this is not included in Bitcoin Core but is included in a fork of Bitcoin Core called "btc1".
By modifying the version field in the block header, miners can "signal" their support for any combination of deployments. Bit 1 of the version field corresponds to SegWit and bit 4 corresponds to SegSignal.
https://blockchain.info/charts/bip-9-segwit charts the signalling of SegWit (bit 1) where each datapoint is in fact the average signalling rate of the prior 2016 blocks (~ 2 weeks). However, as you rightly observe, there has been a recent increase in SegWit signalling. Of the last 144 blocks (~ 1 day), 131 have signaled for SegWit (~ 91%). If the 2016-block moving average chart is at 95% or higher at a difficulty change then SegWit will "lock in".
That's really clear, junan1, thanks. I appreciate "SegSignal" as a name instead of numbers, which are easier to mix up.
SegSignal is already locked in and all nodes following SegSignal will consider invalid any blocks at height 477120 or greater (~ 2017-07-23, 8:00am UTC). Note again that Bitcoin Core does not include code for this deployment (hence the warnings) and so Bitcoin Core will not reject blocks as SegSignal requires.
I understand that it's also a theoretical possibility that those who are currently sending SegSignal may not actually follow through and orphan non-segwit blocks. But I understand that we expect them to do so.