Property Index is for queries involving property constraints - equality, exist/not null check on property.
Oak property Index:
Node of type "oak:QueryIndexDefinition" with:
1. propertyNames Name list of properties for which the index needs to be created.
jcr:title, cq:lastModified, hideInNav property for a page content
cq:ParentPath property for an AEM Asset
2. declaringNodeTypes Name list of node types for which the index is applicable for
3. unique Boolean true
Applicable for queries with unique constraint
This property should be used along with declaringNodeTypes