When does value attribute get added to asp.net checkbox and how to access

Can anyone explain when the value attribute gets added to html for a asp.net checkbox. Running locally for the below checkbox the html returned has no value attribute included, deploying to a test server I get value="on". Is this dependent on the version of IIS, ASP.NET etc?

<asp:CheckBox ID="chkConfirmRevoke" runat="server" />

Is it possible to get to that value from within the code behind? To be clear I'm not asking how to check if the checkbox is checked. I'm asking how to get the "on" string that is assigned to the value attribute.

Thanks