1. The "bookmark" is the *columnstore index original locator* (per "Pro SQL Server Internals" by Dmitri Korotkevitch). This is an 8-byte value, with the columnstore index's `row_group_id` in the first 4-bytes and an offset in the second 4-bytes. 

2. If you use `DBCC PAGE` to look at the non-clustered index, the 8-byte columnstore index original locator appears in the "uniquifier" column of the `DBCC PAGE` output. This shows that a *unique* non-clustered index does not need to include the columnstore row locator, whereas a *non-unique* non-clustered index does. 

The following code creates a columnstore-organized table with a unique and non-unique b-tree nonclustered index on the same column:

    CREATE TABLE dbo.Heapish
    (
    	c1 bigint NOT NULL,
    	c2 bigint NOT NULL,
    	INDEX CCI_dbo_Heapish CLUSTERED COLUMNSTORE
    );
    GO
    INSERT dbo.Heapish WITH (TABLOCKX)
    	(c1, c2)
    SELECT TOP (1024 * 1024 * 8)
        c1 = ROW_NUMBER() OVER
            (ORDER BY C1.[object_id], C1.column_id),
        c2 = ROW_NUMBER() OVER
            (ORDER BY C1.[object_id], C1.column_id)
    FROM master.sys.columns AS C1
    CROSS JOIN master.sys.columns AS C2
    ORDER BY
        c1
    OPTION (MAXDOP 1);
    GO
    CREATE UNIQUE NONCLUSTERED INDEX UNIQUE_c2 ON dbo.Heapish (c2) WITH (MAXDOP = 1);
    CREATE NONCLUSTERED INDEX NONUNIQUE_c2 ON dbo.Heapish (c2) WITH (MAXDOP = 1);

We can see the size of the index row at different levels of the b-tree using [`sys.dm_db_index_physical_stats`][1]:

    SELECT
        DDIPS.index_level,
        DDIPS.page_count,
        DDIPS.record_count,
        DDIPS.min_record_size_in_bytes,
        DDIPS.max_record_size_in_bytes
    FROM sys.dm_db_index_physical_stats
    (
        DB_ID(),
        OBJECT_ID(N'dbo.Heapish', N'U'),
        INDEXPROPERTY(OBJECT_ID(N'dbo.Heapish', N'U'), N'UNIQUE_c2', 'IndexID'),
        NULL, 'DETAILED'
    ) AS DDIPS;
    
    SELECT
        DDIPS.index_level,
        DDIPS.page_count,
        DDIPS.record_count,
        DDIPS.min_record_size_in_bytes,
        DDIPS.max_record_size_in_bytes
    FROM sys.dm_db_index_physical_stats
    (
        DB_ID(),
        OBJECT_ID(N'dbo.Heapish', N'U'),
        INDEXPROPERTY(OBJECT_ID(N'dbo.Heapish', N'U'), N'NONUNIQUE_c2', 'IndexID'),
        NULL, 'DETAILED'
    ) AS DDIPS;

The output is:

[![Unique index][2]][2]

[![Nonunqiue index][3]][3]

Both structures have the same row size at the leaf level, but the nonunique nonclustered index is 12 bytes larger than the unique nonclustered index at the non-leaf levels due to the 8-byte columnstore locator, plus 4 bytes of overhead for the first variable-length column in a row (uniquifier is variable length).

  [1]: https://docs.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-db-index-physical-stats-transact-sql
  [2]: https://i.stack.imgur.com/BpDTa.png
  [3]: https://i.stack.imgur.com/zHcTB.png