BSEG (Accounting Document Segment) is a standard table in SAP R3 ERP systems.
BSEG stores the line items for accounting documents. Accounting documents make up the financials of your organisation. Header lines are stored in table BKPF. The link between BSEG and BKPF is made on fields MANDT, BUKRS, GJAHR, BELNR. The same document number can be used in different company codes. The same document number can also be used multiple times within the same company code in different years.
For accounting documents that have an effect on the financials (i.e. BKPF.BSTAT is blank) total debits should match total credits. Debit versus credit is indicated by field SHKZG.