utm_term was built for one job: recording which paid search keyword triggered an ad, from back when marketers needed a way to pull that detail into a separate analytics tool. Google Ads could auto-fill it using the ValueTrack parameter {keyword}, so the keyword that matched a search query landed in your reports without anyone typing it by hand.
Why it is often empty or auto-filled now
Google Ads enables auto-tagging by default, which appends gclid to the destination URL and lets Google Ads and Analytics share click and keyword data on the backend. That link already carries the keyword match information inside Google's own systems, which makes a hand-set utm_term redundant. Bing Ads works the same way through msclkid. If your campaigns run through either platform with tagging enabled, an empty utm_term is normal. It is not a mistake.
A real value looks like this
When it is set manually it is short, plain text: utm_term=running+shoes or utm_term=budget_travel. There is no character limit enforced by the field itself, just whatever your analytics tool will store and display cleanly.
When it is still worth setting
Set it if you are building links for a paid search platform that does not auto-tag, or if you are pulling reports into a tool that never sees the platform's own keyword data. The field also gets repurposed off search entirely, holding an audience or segment label instead, since there is no keyword to record in that context. That works as a personal convention, but it is not what the field was originally defined to hold, so keep your own usage consistent rather than expecting a platform to enforce a meaning for you. For links with no search intent at all, a utm_content value naming the variant usually does the job better.