what is source in field page snippet

[pmohanakrishna]

field(MyField;# Source)
{
FieldPropertyName = FieldPropertyValue;
}

[freddydk]

On the VM, in the folder C:\DEMO\New Developer Experience\AL you will find some of the base objects as .al files (2 pages, a codeunit and a table). Source is the source expression (typically the same as the field name in the page). One example where they are different is the customized calendar field:
field("Customized Calendar";CalendarMgmt.CustomizedCalendarExistText(CustomizedCalendar."Source Type"::Customer,"No.",'',"Base Calendar Code"))

[pmohanakrishna]

so Its SourceExpr property in page. Thanks..

[pmohanakrishna]

Is this Source property working in preview?
I am trying to create a field like below and is not working

pageextension 70000003 CheckComments extends "Job Card"
{
layout
{
// Add changes to page layout here
addlast(General)
{
field(Comments;ChechComments.CheckCommentsExist(Rec."No."))
{
Editable = FALSE;
}
}
}

var 
  ChechComments : Codeunit 70000000;

}

It added a field but not showing any check box.