Why Intellij doesn't import the TestNG maven dependency automatically ?

Hi,

I opened a Maven project and import all the maven dependencies. 

Everything is fine , except for TestNG dependencies. 

I have this in my POM.xml 

<dependency>
   <groupId>org.testng</groupId>
   <artifactId>testng</artifactId>
   <version>6.14.3</version>
   <scope>compile</scope>
</dependency>

But I see this in my codes

I can see the TestNG appears in the Maven dependencies list , with (test) at the end. Not sure what does it mean. 

Then I saw this options in my editor

When I click "Add library 'Maven: org.testng:testng:6.14.3' to classpath" , it solves my problem straightaway. 

I checked the list of my Maven dependencies, I see the "(test)" disappear now. 

My questions:

1. Why Intellij doesn't add the dependencies automatically ?  It works automatically for other dependencies in the list (javax , mysql, etc), except for this TestNG dependency. Is there something special with this TestNG dependency ? 
2. What does " Adding XXX to classpath" mean?  Does it mean adding to External Libraries list ? 
3. Secondly , in the Maven dependencies list, what does (test)  and (provided) mean ? 

Thanks.