curlの-uと-Uの違い

つい先程、授業で「squidにBasic認証使って使える人制限かけようねー」という内容をやった。
その際、最後の確認の場面でcurlの-uと-Uの違いで悩まされた…。

-u/--user
Specify user and password to use for server authentication. Overrides -n/--netrc and --netrc-optional.

If you use an SSPI-enabled curl binary and do NTLM authentication, you can force curl to pick up the user name and password
from your environment by simply specifying a single colon with this option: "-u :".

If this option is used several times, the last one will be used.

-U/--proxy-user
Specify user and password to use for proxy authentication.

If you use an SSPI-enabled curl binary and do NTLM authentication, you can force curl to pick up the user name and password
from your environment by simply specifying a single colon with this option: "-U :".

If this option is used several times, the last one will be used.

マニュアル読んでみたけど、違いと言ったら「Overrides -n/--netrc and --netrc-optional.」のとこくらい。
一体何が違うんだ！ずっと-u使い続けて、407 ERRORに悩まされてたよ！
帰ったらじっくりソース読んでみるか…。