HDMI 2.1a adds Source-Based Tone Mapping, or SBTM. SBTM is a new HDR feature that offloads some of the HDR tone mapping to the content source (like your computer or set-top box) alongside the tone mapping that your TV or monitor is doing.
SBTM is intended to help existing HDR setups work better by letting the content source better optimise the content it passes to the display or by removing the need to have the user manually calibrate their screens for HDR by having the source device configure content for the specific display.
It could also be used when there's a mix of content types, like for streamers (who could have an HDR game playing alongside a window of black and white text), displaying each area of content.
But like every other unique HDMI 2.1 headache, including variable refresh rates, automatic low latency connections, and the bandwidth necessary to offer things like 10K resolution or 120Hz refresh rates, SBTM will be an optional feature that manufacturers can support if they want.
This means that, like every HDMI 2.1 feature, some will adopt it and others will not so you will be completely unaware if it is available until you try to set it up and it is borked.
To make matters worse all new ports will, in theory, be labeled HDMI 2.1a. The HDMI Forum’s argument is that this is always how its standards have worked, and that optional features allow manufacturers to have flexibility in what functionality they offer (an entry-level set, for example, probably doesn’t need ports that support 8K 120Hz VRR gaming). And the group says companies are required to list what features their hardware supports so that it's clear to customers what their hardware is capable of, beyond the number expectation.
However most sane people this flies in the face about what is meant by the word “standard” which implies that everything conforms to a set number of criteria. HDMI 2.1a could have included in its standard the ability to turn the user into a pink unicorn. Users wanting to be turned into pink unicorns would, of course, be disappointed.