Problem with Tomcat9 Context's docBase attribute

I am new to Tomcat deployment.

I found in Tomcat's document and it says this about <Context> tag's docBase attribute:

The value of this field must not be set unless the Context element is defined in server.xml or the docBase is not located under the Host's appBase.

Here is my problem:

1. I create a <Host> tag inside server.xml and set appBase="C:/xxx/"

2. create a xml file named ROOT.xml inside C:/xxx/ and add <Context docBase="../yyy" />

3. put index.html inside C:/yyy/

Why is it doesn't work when I try to browser localhost:8080/ ?

Doesn't it supposed to show the index.html inside C:/yyy ?