It truncates both the record contents and the domain name, which means that if you've got a bunch of long domains with TXT records, telling them apart means copying the contents into a text editor to see the truncated text.
e.g
somekey._dkim.subdomaina.domain.com k=rsa; t=s; p=MIGfAAAA
somekey._dkim.subdomainb.domain.com k=rsa; t=s; p=MIGfBBBB
somekey._dkim.subdomainc.domain.com k=rsa; t=s; p=MIGfAAAA
getting truncated down to somekey._dkim.subdo... k=rsa; t=s; p=MI...
somekey._dkim.subdo... k=rsa; t=s; p=MI...
somekey._dkim.subdo... k=rsa; t=s; p=MI...
Compared to AWS, which is visually unappealing, but I can actually see my DNS records....