# 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).
## Data Type Available data types - String: - Boolean - Integer - Double - Date and time - Date - Time - HEX-encoded binary - URI - Year-month duration - Day-time duration - Base 64 binary - X. 500 name - RFC822 name --- [*https://docs.oasis-open.org/xacml/2.0/access\_control-xacml-2.0-core-spec-os.pdf*](https://docs.oasis-open.org/xacml/2.0/access_control-xacml-2.0-core-spec-os.pdf)