I do not know if this question is still of any interest to you.
Basically in standard application SIA , PAR_BU-ID is not used, so if you have data in this column, you need to either consult customization documentation or use Siebel Tools to identify Field and BC mapped to this column so you should be able to better understand the functional meaning of this column.
BU_ID: The business unit (Internal Organization) to which this Account (or other organization) belongs.
PAR_BU_ID: Indicates the Business Unit that this Organization Supports; Business Unit that this Organization is providing Sales or Service for. This column defines "Org" visibility for positions of this division.