reserved OAuth2 client credential for SAML?

[santhoshkumar_n]

I am going through the following document to call apigee management APIs.

As its SAML enabled org, have the machine user and credentials.

In this documenation, to get Machine users OAuth2 tokens,

Ref : https://docs.apigee.com/api-platform/system-administration/automating-tasks, it was mentioned

For authorization, pass a reserved OAuth2 client credential in the Authorization header. The call prints the access and refresh tokens to stdout.

What is meant by "reserved OAuth2 client credential and how to get it?

Also

curl -H "Content-Type: application/x-www-form-urlencoded;charset=utf-8" \
  -H "accept: application/json;charset=utf-8" \
  -H "Authorization: Basic ZWRnZWNsaTplZGdlY2xpc2VjcmV0" -X POST \
  https://zoneName.login.apigee.com/oauth/token -s \
  -d 'grant_type=password&username=machine_user_email&password=new_password'

why we need Authorization header as already passing username and password?

Much appreciated your help.