Button 'Confirm' sometimes is disabled, sometimes it is enabled

Hi all,

If I change some data in Purchase Order Header, sometimes button Confirm is enabled sometimes it is disabled.

For example:

- I change payment terms in PO Header, button Confirm is enabled.

- I change delivery terms in PO Header, button Confirm is disabled.

Is it Dynamics AX standard features? any documentation which fields (data) that can cause button Confirm is enabled if there is change for that fields?

BR,

Tanto Widjaya