Split option 6 used between RU and DU

Hi O-RAN experts.

Please tell split option 6 used between RU and DU.
Is it for small cell as RU used then split option 6 valid.

Is this understanding correct?

It’s suitable for small cells as per Oran alliance notes.

5 and 6 summary:

Option 5 (Intra MAC split)
Option 5 assumes the following distribution:
RF, physical layer and lower part of the MAC layer (Low-MAC) are in the Distributed Unit
Higher part of the MAC layer (High-MAC), RLC and PDCP are in the Central Unit
Therefore, by splitting the MAC layer into 2 entities (e.g. High-MAC and Low-MAC), the services and functions provided by the MAC layer will be located in the Central Unit (CU), in the Distributed Unit (DU), or in both. An example of this kind distribution given below.

In High-MAC sublayer the centralized scheduling in the High-MAC sublayer will be in charge of the control of multiple Low-MAC sublayers. It takes high-level centralized scheduling decision. The inter-cell interference coordination in the High-MAC sublayer will be in charge of interference coordination methods such as JP/CS CoMP.
In Low-MAC sublayer the time-critical functions in the Low-MAC sublayer include the functions with stringent delay requirements (e.g. HARQ) or the functions where performance is proportional to latency (e.g. radio channel and signal measurements from PHY, random access control). It reduces the delay requirements on the fronthaul interface. Radio specific functions in the Low-MAC sublayer can for perform scheduling-related information processing and be reporting. It can also measure/estimate the activities on the configured operations or the served UE’s statistics and report periodically or as requested to the High-MAC sublayer.

Option 6 (MAC-PHY split): The MAC and upper layers are in the central unit (CU). PHY layer and RF are in the DU. The interface between the CU and DUs carries data, configuration, and scheduling-related information (e.g. MCS, Layer Mapping, Beamforming, Antenna Configuration, resource block allocation, etc.) and measurements.

Here you can find more details:

What are 5G different functional split options? And why we need it?

Ok thanks it means option 6 valid when RU is small cell.
We can’t use option 7.2, 8 with small cell.

Yes, exactly.