# Expressions ## Description > The Expression signifies that an element that extends the ExpressionType and is a member of the Expression substitution group shall appear in its place. The Expression is not used directly in a policy. Expressions are elements that allow to evaluate conditions within rules and policies to make access decisions. ## Related objects - [**Variables**](https://bookstack.soffid.com/books/xacml/page/variables "Variables") - [**Conditions**](https://bookstack.soffid.com/books/xacml/page/conditions "Conditions") ## Standard attributes The attributes depend on the Expression type selected.
**EXPRESSION TYPE** | **OTHER FIELDS** | **DATA TYPE** |
Attribute value | **Value**: alfanumeric field | [Available data types](#bkmrk-%C2%A0-0). |
Resource | **Attribute designator** - URL - Soffid object - Account name - System name - Login name - Vault folder - Access level | [Available data types](#bkmrk-%C2%A0-0). |
Subject | **Attribute designator** - User - User attributes - Account - System - Role - Group - Primary Group - IP Address | [Available data types](#bkmrk-%C2%A0-0). |
Action | **Attribute designator**: - method | [Available data types](#bkmrk-%C2%A0-0). |
Environment | **Attribute designator**: - Country - Current Time - Current Date - Current DateTime | [Available data types](#bkmrk-%C2%A0-0). |
Attribute selector | **Attribute selector**: alphanumeric field | [Available data types](#bkmrk-%C2%A0-0). |
Variable | **Variable**: alfanumeric field | -- |
Function | **Function type**: - Comparison - Arithmetic - Conversions - Date conversions - Boolean Operators - String Functions - Set Functions - Bag Functions - HigherOrderBagFunctions - XPath | [Available data types](#bkmrk-%C2%A0-0). |
Function name | **Function type**: - Comparison - Arithmetic - Conversions - Date conversions - Boolean Operators - String Functions - Set Functions - Bag Functions - HigherOrderBagFunctions - XPath **Function**: the value depends on the function type selected. | [Available data types](#bkmrk-%C2%A0-0). |