Google Ads users spotted a label in the Google Ads demographics a new tab that said "Signal". Ginny Marvin from explained on Twitter:
It indicates optimized targeting is enabled. OT uses audience & demo inputs as signals to find more conversions w/in campaign goal, so you'll see impressions outside of your set targeting signals. To help distinguish, set signals are labeled as "Signal."
Optimized targeting, according to the help document "grants your campaign flexibility to explore which customers have the highest likelihood to convert within your campaign goals.