Q:

where we use immediate="true"

so i have a
A.xhtml
<p:tabView id="headOfAccountsId_tabview">
<p:tab title="Main Head Of Accounts" id="mainHeadOfAccountsId_tab">
 // completely finished with codes 
 // here i have done some inputtext fields with jsf validation , 
 // and submit button to call action on ajax
 // and p:datatable to view only records and user can edit and delete record on ajax
</p:tab>
<p:tab title="Sub Head Of Accounts" id="subHeadOfAccountsId_tab">
 // just have open and closing tags of p:tab i meant i will do code later here 
</p:tab>
</p:tabView>

and i have an other xhmt file for different purpose on that i am using
B.xhtml
 // all the input fields outside the <p:tabView >
 // here i have done some inputtext fields with jsf validation , 
 // and submit button to call action on ajax
<p:tabView >
<p:tab>
 //p:datatable to view only records
</p:tab>
<p:tab>
//p:datatable to view only records
</p:tab>
</p:tabView>

my question is that in both files (A and B) i haved used
<p:ajax event="tabChange" />

but in file A.xhtml when i changed the tab listener does not call method(action method of java class) without
  immediate="true"

and in file B.xhtml when i am changing tabs  method is calling without
  immediate="true"

can any one makes me understand whats happening ? or why/where we use 'immediate' attribute ? 
should i post my whole code ? 

A:

immediate="true" causes the actions to be invoked in the 'Apply Request Values' phase before any validations/conversions. The tag defaults to false, which makes JSF go through the standard lifecycle and call the actions at 'Invoke Application' phase. 
I'm guessing (difficult to get it from your stub example) that you have validation or conversion errors. On an error event, JSF skips the remaining lifecycle phases, so the tabChange event won't be called.
You should try debugging it by creating a <h:messages /> tag on top of the page or checking the stack trace if its logged.
For more information please refer to:
http://www.javacodegeeks.com/2012/01/jsf-and-immediate-attribute-command.html
http://docs.oracle.com/javaee/6/tutorial/doc/bnaqq.html
http://balusc.blogspot.hu/2006/09/debug-jsf-lifecycle.html#AddImmediateTrueToUIInputOnly

