本文综合baidu与bing返回结果。

https://www.supermicro.org.cn/support/faqs/faq.cfm?faq=16169

Question：
For LSI Host Bus Adapter (HBA) firmware, what is the difference between IT mode and IR mode?
对于LSI主机总线适配器（HBA）固件，IT模式与IR模式有何区别？

Answer:
The two firmware types are Initiator Target (IT) and Integrated RAID (IR). IR is the typical hardware RAID firmware that the internal cards or boards ship with.
The IT firmware, on the other hand, is the Initiator Target firmware which is designed to allow a SAS HBA to connect to up to 256 devices using SAS Expanders. The IT firmware is typically shipped on external cards. The IT firmware does not have any RAID capabilities and simply acts as an HBA.

这两种固件类型是启动器目标（IT）和集成RAID（IR）。
IR是内部卡或板附带的典型硬件RAID固件。
IT固件允许SAS HBA使用SAS扩展器连接到多达256个设备。IT固件通常在外部卡上提供。IT固件没有任何RAID功能，只是充当HBA。

LSI SAS3008是常用的RAID卡种类。此RIAD卡有两种固件：IT和IR。
当固件是3008IT时，只能直连硬盘而不能配置RAID。
当固件是3008IR时，可以配置RAID。

Basicly the IT firmware is only just a HBA and the boot up time is quick. The IR firmware is the raid software firmware so boot up is a bit longer than having the IT firmware only. You can cross flash with the LSI firmware in IT mode only or IT and IR mode which is default in LSI. If you are going with pure software raid then the IT firmware is best. 

更多解释：

LSI-RAID卡命名后缀的含义（it-ir-imr-mr）（华为）

broadcom官方MR、iMR、IR和IT软件功能比较矩阵