I created 2 lists today. Don't know why both lists become default list and not removable. Is there any way I can delete it from my lists? And what makes it a default list?
You are not able to have more than one default list, so I am not sure why you would not be able to remove a list if there are more than one in the account. It is a default list designated when you are in the Join My Mailing list area of the user interface (in the contacts tab). It is the list that is selected by default when a user goes to the Join my mailing list form. It cannot be deleted until a different list is changed to the default list.
If this is not what you mean then I may not be clear on the question.
I would like to give you an update on this issue. I did some further testing and I do see that if you set the OptInDefault to true when creating the list XML, it does allow you to have more than one default list. This was not intended to work this way and I have alerted our engineers to this.
Currently there are two ways you can fix this. If you do a PUT call on all of the lists that have been set to default in the API, and reverse the XML to set that field as false, that will work. You can also log into the user interface and go to the Contacts tab, and then click on Join My Mailing list, and edit the Join My Mailing list Form. If you click on Lists link, and select a different default list, it will remove the default list tags from every list but the new list you selected.