Naive Question: facet minLength defaultvalue for xs:string

Hello.

I have a real simple question.
The minLength facet in for xs:string is optional.
If omitted, the default value for minLength is zero.

My question is: Does anyone has a link into the W3C universe where this
default/behaviour is defined?
In
http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#rf-minLength
I didn’t find any hint.
Also
http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/#string
give no answer to my question.

Thx for any reference
Christian