Dear Shri Shankar Nath,
It is up to each Member to decide how to structure their national WIGOS Station Identifiers. Some Members use different 3rd block for different organizations. However, it's not recommended to describe the observation type in the 3rd block, as it can make it difficult to maintain it in the future, for example if a station type is converted to another type of observation or if a station performs some different observation types.
Members may decide whether the implementation and maintenance of the national schema for allocating WSIs is centrally managed by a single organization or is distributed across various national organizations receiving delegated ranges for assigning WSIs.
Further information and references on WSI are available in the WIGOS community webpage, https://community.wmo.int/activity-areas/WIGOS/implementation-WIGOS
I hope this helps.
Thank you, kind regards