> For the complete documentation index, see [llms.txt](https://docs.rainbird.ai/rainbird/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rainbird.ai/rainbird/knowledge-modelling/modelling-features/rblang-reference/rules-and-expressions.md). # Rules and expressions ### Rule structure Rules are relationship instances with nested conditions. ```xml ``` Use these rule attributes: * `type` — the relationship being inferred * `object` — the inferred value when it is fixed * `cf` — certainty factor * `alt` — alternative evidence text shown in the evidence tree * `name` — optional rule name Rule names do not need to be unique. Keep them under 170 characters. ### Evidence text Rule attributes and condition attributes support `alt`. In Studio, `alt` is called **evidence text**. Use it to return a clearer explanation in the evidence tree. That explanation appears alongside the fact. It can be shown instead of the raw fact text. Variables can be used in `alt` text, but they must be wrapped in `{{ }}`. ```xml ``` ### Variables Variables start with `%`. * `%S` — the current subject * `%O` — the output object * custom names such as `%AGE` or `%RESULT` Use relationship conditions to bind values. ```xml ``` Use expression conditions to test or calculate values. ```xml ``` ### Critical expression rules {% hint style="warning" %} `=` is only for comparison. Never use `=` for assignment. {% endhint %} Correct assignment: ```xml ``` Invalid assignment: ```xml ``` ### String comparisons Use single quotes around string literals. ```xml ``` ### Missing facts Do not test for `null` directly. Use `countRelationshipInstances()` instead. ```xml ``` ### Multi-step calculations Break complex logic into separate steps. ```xml ``` ### Condition weighting and behaviour Conditions can influence certainty and success. ```xml ``` * `mandatory` — must be satisfied * `optional` — adds confidence when satisfied ### Authoring guidance Keep each rule focused on one result. Write rules for the general case. Do not hard-code output values that only fit one example dataset. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.rainbird.ai/rainbird/knowledge-modelling/modelling-features/rblang-reference/rules-and-expressions.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.