What does base: true mean in the cluster template json file of a CDP cluster ?

[#1]

I am trying to import a cluster template json file to form a cluster in CDP. I am unclear about the meaning of "base: true" in the service role definition blocks. 

{

"refName": "impala-IMPALAD-BASE",
"roleType": "IMPALAD",
"base": true
},

It is not documented on CDP website either.