How drupal keeps revisions for some content types only

Question

In Drupal 6 for some content types when editing a node its replicated and keeps revisions for that content type by keeping vid (version id) for that in the table, But for some other content types it wont make new table entries when editing that content types . How is it like that ? Or is there any criteria to keep revisions for some content type only ?

update from comment

The revision information can be done when creating a content type item itself , but how come its auto selected for some content types ?

2nd update

When creating a new content type i can see the options as follows :

But as you can see , revision settings is inside submission form settings but **its disabled** here , how i can set revisions for particular content type only for some content type only .

Answer 1

For D7:

admin/structure/types/manage/YOUR_CONTENT_TYPE

You need to tick the checkbox "Create new revision" for saving revisions of any content type you want.

For D6:

administer/content management/content types/edit/workflow

Thanks!