The Babylon Portfolio Segment — the Ledger for a Group of Positions

By Gary Kennedy
October 16, 2025

Segment — the ledger for a group of positions.
A Segment is one of Babylon’s core concepts. It defines the natural partition of a portfolio, ensuring every security — and all of its trades — belong to one, and only one, Segment. This simple rule keeps capital gains and performance calculations coherent, consistent, and correct at scale.

A Segment can represent a single security, an entire portfolio, or somewhere in between. Its defining rule is completeness: if a security appears inside a Segment, all transactions for that security are included — never split across boundaries. This creates a natural and unambiguous partition of the portfolio, where each Segment forms a self-contained ledger.

That structure is what allows Babylon to compute valuations and realised gains coherently. Because each security’s full trading history resides in one Segment, cost basis and performance can be calculated without overlap or omission.

What happens in practice?

In practice, Segments often reflect how brokerage accounts are structured. These accounts are usually restricted to a specific market or exchange — for example, one account might only trade UK-listed securities, another only South African equities.

Groups of brokerage accounts that share the same restriction naturally form Segments. If investors held only one account per restricted market, Segments would be unnecessary — the brokerage account itself would already define the boundary.
But in reality, investors maintain multiple accounts within the same market — perhaps across different brokers or account types.

Babylon unifies these related accounts into a single Segment. It’s why Segments often carry names like MyUKSegment or MySASegment — each one a self-contained ledger for a particular market domain.

Why the word Segment?

We considered several alternatives: Ledger, Book, Domain, even Partition. Each captured part of the idea but not all of it.

• Ledger conveyed the accounting foundation, but it felt too general — it didn’t express the idea of exclusivity.
• Book was close to trading terminology (“equity book”, “FX book”), but its meaning varies across systems.
• Domain and Partition were mathematically accurate, yet abstract and sterile — too far from the language of portfolios.

The technically precise name might well be PositionGroupLedger — accurate, but far too heavy. We wanted a single word that felt natural in both technical and financial contexts: one that conveyed structure, separation, and the essential rule that if a security appears in the ledger, all of its transactions must be there too.

We chose Segment because it does exactly that. It signals a portfolio divided into clearly bounded, internally consistent pieces, each standing as its own ledger.

This rule — all transactions for a security belong together — is the quiet foundation behind Babylon’s analytics. It ensures that every gain, every position, and every report remains internally consistent and logically complete.