How to understand memory region with strict or relaxed order?

[XCloudAI]

How to understand strict & relaxed order? What's the difference between them?

typedef enum {
    /* Default memory region with either strict or relaxed order */
    UCT_IB_MR_DEFAULT,
    /* Additional memory region with strict order,
     * if the default region is relaxed order */
    UCT_IB_MR_STRICT_ORDER,
    UCT_IB_MR_LAST
} uct_ib_mr_type_t;