the purpose of filmic or sigmoid is to squeeze a unlimited scene referred tonal range into a limited display referred range.
There’s a quite good post on using filmic here: Let's learn Filmic RGB! Your one stop shop to understanding filmic-based approach to edits! - #33 by s7habo
and darktable's filmic FAQ
even these posts focuses on filmic that’s also relevant for sigmoid …
It’s worth to spend some time for those posts and threads
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