According to the Hardware Description: Bbu5900 V100r013c10, these statements are correct about the space principles for 5G BBU installation:

If the installation space is insufficient, the BBU can be installed on a wall. Ensure that the wall meets the load bearing requirements: The BBU can be installed on a wall using wall-mounted brackets. The wall must be able to bear at least four times the weight of the BBU and must be flat and vertical. The installation height of the BBU must not exceed 2 m.

If the space is greater than 600 mm, a cabinet can be added to install the BBU: The BBU can be installed in a cabinet that meets the requirements of Huawei base stations. The cabinet must have a width of 19 inches and a depth of at least 600 mm. The cabinet must also provide sufficient ventilation, grounding, and lightning protection for the BBU.

If the remaining space of an existing cabinet is greater than 4 U, the BBU can be installed directly: The BBU can be installed in an existing cabinet that has enough space for it. The BBU has a height of 2 U, so it requires at least 4 U of space in a cabinet. The existing cabinet must also meet the requirements of Huawei base stations.

The other statement is incorrect about the space principles for 5G BBU installation:

If the installation space is insufficient, install 5G boards directly into the existing 4G B8U: This statement is not supported by Huawei base stations. The 5G boards cannot be installed directly into the existing 4G B8U because they have different slot distributions and board types. The 5G boards can only be installed in a BBU5900 subrack that supports both LTE and NR modes.