Rules and evidence
Where every number comes from: the agency guides as signed rule releases, the lender overlays, and the evidence behind each fact.
Updated 2026-09-26
Rules and evidence
Every number on the page is a rule with a source, run over a fact with a page.
A rule is a record
A rule is data. Each one carries:
- the authority: Fannie Mae, Freddie Mac, HUD, the CFPB, or the lender
- the program and product it applies to
- the fact it tests, or the calculation it performs
- its parameters, each with its source and effective date
- the citation: guide, section and page, and the version of the guide it was read from
- its own cases: one that passes, one that fails, and one on the boundary
B3-3.1-01The evaluators are few and are code: a threshold, a ratio, a history average, a gross-up, an add-back cash flow, a documentation requirement, a computed value. The rules are many and are data, so a rule can be diffed when a guide changes and tested through one runner.
The sources
| Source | What it governs |
|---|---|
| Fannie Mae Selling Guide and Eligibility Matrix | Conventional income, ratios, LTV by product and occupancy, reserves and documentation; the DU side |
| Freddie Mac Seller/Servicer Guide | The same, for LP |
| HUD Handbook 4000.1 | FHA income, ratios and mortgage insurance, for an FHA loan or a switch to one |
| County loan limits | The conforming and FHA limit for each county and number of units, from HUD's published limit files |
| Regulation Z and Regulation B | Points and fees under the qualified-mortgage cap, loan originator compensation, and the notices a changed loan can require |
| DU and LP release notes | Changes to what each AUS states and requires, diffed against the stored version |
| UWM public product terms | Minimum credit score, maximum LTV and DTI, reserves and product restrictions as printed on uwm.com, each with its retrieval date |
| Lender overlays | A rule that references the agency rule it tightens; the result cites both |
Before the AUS, and after it
The same rules run twice. Before any AUS run, every published limit is checked against the loan's terms as the loan file states them, with the arithmetic and the citation. A figure only the AUS judges, such as LP's DTI and reserves, is shown and never passed. After a run, the AUS's answer leads and the loan file's own checks sit beside it. An approval is never second-guessed, and nothing is called approved until DU or LP has said so.
Releases
Proxara fetches the public guides, the bulletins, the handbook and the release notes, diffs each against its stored version, verifies every quotation against the source, runs every case, and signs the release. Your environment verifies each release locally and recomputes the open loans a change touches, keeping the earlier result beside the new one. Every result records the rule versions it ran under. Coverage and effective dates are shown on the page; they never stop a calculation.
Evidence
Every fact says where it came from: a document fact, the page and the words on it; a report fact, the report and the line; a loan-file fact, the field in the MISMO file; a worked-out figure, its arithmetic from the facts it used. Code checks the reader's quote against the page before a value is kept.
DU findings reports and LPA feedback certificates in their published layouts are read by code from the report's own text, every value anchored to its words, in a fraction of a second. A scan, or a layout code does not recognise, is read by the model and checked the same way.
How the engine is checked
The expected value for every supported case is derived by hand from the guide, with the section cited, and the engine is corrected to it. For self-employment, Fannie Mae's public Income Calculator is the outside check. Every result records the model and rule build that produced it.