The credit line is the text a publisher should print next to a photo, for example Jane Doe / Northlight Agency. It is stored in IPTC dataset 2:110 (Credit) and in XMP as photoshop:Credit; IPTC Core calls the field Credit Line. Despite the overlap it is not the same thing as Creator. The creator is who took the picture. The credit line is how they, or their agency, want to be acknowledged, and it often names an agency or a client.
Google lists Credit Line among the IPTC fields it extracts for Google Images, next to Creator and Copyright Notice, and can show it in the image details. Its documentation doesn't say whether it reads the IIM copy or the XMP one. Fill both.
The two copies don't behave the same way. IIM caps Credit at 32 bytes, which runs out quickly once an agency name is involved, and ExifTool enforces the limit:
$ exiftool -IPTC:Credit="Photo by Jane Doe for Northlight Agency Paris" photo.jpg
Warning: [Minor] IPTC:Credit exceeds length limit (truncated) - photo.jpg
1 image files updated
$ exiftool -IPTC:Credit photo.jpg
Credit : Photo by Jane Doe for NorthlightXMP has no such limit, so the IIM and XMP values can quietly drift apart. Either keep the line under 32 bytes (accented letters take two bytes in UTF-8) or accept that IIM-only readers see a truncated version. Shorter is better anyway; nobody prints "Photo by" in a credit.
Format matters too. A WebP file has no IPTC block at all, so if you publish WebP, the XMP copy is the only one that can come along.
Both IPTC:Credit and XMP:Credit are in our whitelist. The EXIF editor writes them on JPEG and TIFF. It rejects &, +, commas and colons among other characters, so "Jane Doe / Northlight Agency" goes through and "Smith & Co" does not. The ExifTool version:
exiftool -IPTC:Credit="Jane Doe / Northlight Agency" -XMP-photoshop:Credit="Jane Doe / Northlight Agency" photo.jpgThe credit line usually sits next to a copyright notice; the guide to adding copyright to photos sets the rights fields together.