Cover index in Couchbase

How does couchbase decided to cover an index. I have created two indexes but when i look into execution plan one is covered and other is not.

created index idx_123 on bucket( value1,value2,value3,value4,value5,value6) where type=“ABC”

Create index idx_324 on bucket (a,b,c,d) where type=“PQR”

What metrics does it consider for taking this decision?

If index has all the information that required by query then it uses covered index.
If plan uses UnionScan, IntersectScan will not cover the query.

so my query is
Select a,b,c,d from bucket where type=PQR and
and b=""
group by c,d.

But i can see in plan it is not getting covered though index has all data

PQR must be string.

Select a,b,c,d from bucket where type="PQR" and
and b=""
group by c,d

If you have issue post the full EXPLAIN and index definition.

yes let me share execution plan