SQL Server - Bulk load blocking queries on heaps SQL Server books online say in SET TRANSACTION ISOLATION LEVEL:

&quot;Optimized bulk load operations on heaps block queries that are running under the following isolation levels:
SNAPSHOT
READ UNCOMMITTED
READ COMMITTED using row versioning&quot;.

What does optimized bulk load mean here exactly?
Optimized bulk load means SQL Server is using a special bulk load code path (fast load) and minimal logging.
These concepts are described in:

SQL Server 2016, Minimal logging and Impact of the Batchsize in bulk load operations
Prerequisites for Minimal Logging in Bulk Import
The Data Loading Performance Guide

From that last link:

Although readers under RCSI are not affected by X locks, there are two bulk load situations that will block RCSI queries (as well as NOLOCK queries) in SQL Server 2008:

When populating a heap with TABLOCK using BULK INSERT, INSERT-SELECT, or other bulk load operations. This is because the heap load acquires a BULK OPERATION intent exclusive (IX) lock and the NOLOCK or RCSI readers acquire a BULK OPERATION shared (S) lock. To bulk load a heap and permit concurrent readers even using RCSI or NOLOCK, you must eliminate the TABLOCK hint and thus give up minimal logging.