What is the difference between the ValidateClientAuthentication method and the GrantResourceOwnerCredentials method in OAuth OWIN?

Question

What is the difference between the ValidateClientAuthentication method and the GrantResourceOwnerCredentials method in OAuth OWIN?

I am starting to use oauth and owin in .NET. I tried to understand these ValidateClientAuthentication methods and the GrantResourceOwnerCredentials method. I realized that the GrantResourceOwnerCredentials method can be used to verify credentials and generate a token. Then what is the purpose of the ValidateClientAuthentication () method. kindly advise me about this. Many thanks.

 public override Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
        {
            return Task.Factory.StartNew(() =>
            {
                var userName = context.UserName;
                var password = context.Password;
                var userService = new UserService(); // our created one
                var user = userService.ValidateUser(userName, password);
                if (user != null)
                {
                    var claims = new List<Claim>()
                    {
                        new Claim(ClaimTypes.Sid, Convert.ToString(user.Id)),
                        new Claim(ClaimTypes.Name, user.Name),
                        new Claim(ClaimTypes.Email, user.Email)
                    };
                    ClaimsIdentity oAuthIdentity = new ClaimsIdentity(claims,Startup.OAuthOptions.AuthenticationType);


                    var properties = CreateProperties(user.Name);
                    var ticket = new AuthenticationTicket(oAuthIdentity, properties);
                    context.Validated(ticket);
                }
                else
                {
                    context.SetError("invalid_grant", "The user name or password is incorrect");
                }
            });
        }
        #endregion

        #region[ValidateClientAuthentication]
        public override Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
        {
            if (context.ClientId == null)
                context.Validated();

            return Task.FromResult<object>(null);
        }
        #endregion