What is <apex:param> tag in Salesforce?

Hi Udit,

A param i.e. a parameter used for the parent component.

It can be a child of a following components:

<apex:actionFunction>
<apex:actionSupport>
<apex:commandLink>
<apex:outputLink>
<apex:outputText>
<flow:interview>

For e.g. Using <apex:param> in <action:function> given below:

<apex:page controller="ApexParam1Controller">
    <apex:form >
        <a href="javascript:jsActionFunction('TestValue');">Send Param To Apex</a> <br />
        
        <apex:outputPanel id="vfParamPanel">
            The value of parameter is : {!vfParam}
        </apex:outputPanel>
        
        <apex:actionFunction name="jsActionFunction" action="{!doActionFunction}" reRender="vfParamPanel">
            <apex:param name="vfParam" value="" assignTo="{!vfParam}" />
        </apex:actionFunction>
    </apex:form>
</apex:page>

public class ApexParam1Controller{
    public String vfParam{get; set;}
    public ApexParam1Controller(){
        
    }
    public PageReference doActionFunction(){
        // Do Something...
        System.debug('vfParam : ' + vfParam);
        return null;
    }
}