---
title: CasSegment
summary: API reference for CasSegment.
status: draft
owner: SAIF Doc Gen
last_reviewed: 2026-09-15
library: SAIF.Framework.Common.Files
reference_type: package
---

# CasSegment

`CasSegment` models the X12 CAS claim-adjustment segment used in 835 remittance advice transactions to report why a claim or service line's payment differs from the billed amount. It carries a group code plus a repeating series of reason-code/amount/quantity triplets, one triplet per distinct adjustment reason applied to the claim or line.

## Purpose
- Represents the CAS segment identity via `SegmentId` returning the literal `"CAS"` ([BR-020](../../business-rules.md#br-020-first-element-is-the-segment-identifier)).
- Carries `GroupCode` classifying the category of adjustment (e.g. contractual, patient responsibility) applied to the claim/line.
- Carries repeating reason-code/amount triplets in the parallel `Reasoncode`/`Amount` lists, one triplet per adjustment reason, starting at element 2 and advancing by three elements per triplet; the triplet count is `floor((elementCount + 1) / 3)` on `Fill()`.
- Adjustment amounts are exchanged unscaled (not multiplied/divided by 100), unlike the equivalent triplets on `CtxSegment` ([BR-050](../../business-rules.md#br-050-ctx-amounts-scaled-to-cents-cas-amounts-unscaled)).
- Omits a reason-code triplet from `Build()` output entirely when its reason code is empty ([BR-059](../../business-rules.md#br-059-optional-segment-elements-written-only-when-populated)).
- The quantity position of each triplet is not implemented — quantities are neither read nor written ([BR-061](../../business-rules.md#br-061-cas-and-ctx-adjustment-triplets)).

## Context
- Namespace: `SAIF.Common.Files.Edi.Segments`
- Assembly/package: `SAIF.Framework.Common.Files`

## Public members
| Member | Signature | Purpose |
| --- | --- | --- |
| SegmentId | `string SegmentId { get; }` | Returns the literal X12 segment identifier `"CAS"`. |
| GroupCode | `string GroupCode { get; set; }` | The adjustment group/category code (CAS01), element 1. |
| Reasoncode | `List<string> Reasoncode { get; set; }` | The parallel list of adjustment reason codes, one per triplet, at elements 2, 5, 8, 11, 14, 17 (up to six triplets) ([BR-061](../../business-rules.md#br-061-cas-and-ctx-adjustment-triplets)). |
| Amount | `List<decimal> Amount { get; set; }` | The parallel list of adjustment amounts corresponding to each `Reasoncode` entry, at elements 3, 6, 9, 12, 15, 18, exchanged unscaled ([BR-050](../../business-rules.md#br-050-ctx-amounts-scaled-to-cents-cas-amounts-unscaled)). |
| Build | `IEdiSegment Build()` | Creates a `SegmentBuilder("CAS")`, adds `GroupCode` at element 1, then for each populated `Reasoncode` entry adds the reason code and matching `Amount` at the triplet's two positions, skipping triplets whose reason code is empty ([BR-059](../../business-rules.md#br-059-optional-segment-elements-written-only-when-populated)). |
| Fill | `void Fill(IEdiSegment segment)` | Computes the triplet count as `floor((elementCount + 1) / 3)` and, for each triplet index, assigns `Reasoncode[i]` and `Amount[i]` from the segment's elements ([BR-061](../../business-rules.md#br-061-cas-and-ctx-adjustment-triplets)). |
| Parse | `static CasSegment Parse(IEdiSegment segment)` | Instantiates a new `CasSegment`, calls `Fill(segment)` on it, and returns it. |

## Behavioral notes
- `Fill()` assigns into `Reasoncode[i]` and `Amount[i]` by list index without first allocating those lists to the required size; because `List<T>` indexer assignment requires an existing element at that index, calling `Fill()` on any segment that actually carries adjustment triplets throws an `IndexOutOfRangeException`/`ArgumentOutOfRangeException` unless the caller has pre-sized both lists — this appears to make `Fill()` unreachable for any segment carrying adjustments as currently written**.
- Similarly, `Build()` requires the caller to have already populated `Reasoncode` and `Amount` with matching-length entries before calling it, since it does not allocate or validate them itself**.

## Constraints and usage notes
- Callers must manually pre-populate `Reasoncode` and `Amount` with the correct number of entries before invoking either `Build()` or `Fill()`, an usage requirement not enforced or documented by the class itself**.

> **Note:** Statements marked with `**` could not be confirmed; treat
> as unknown until verified.

## Sources
- Common\SAIF.Common.Files\Edi\Segments\CasSegment.cs