Default social accounts
A tick to the left of one of your social accounts, indicates that it is set as a default. Default social accounts will be automatically selected whenever you open the composer to schedule a post, when you first login to Buffer (more info below). If you often schedule posts for a specific set of social accounts, it might be helpful to mark those as defaults. A default social account can always be deselected in the composer if you don't wish to include it.
If you'd rather pick and choose social accounts each time you open the composer, you might prefer to remove all the ticks to the left of your social accounts, thus making them all non-defaults.
Default social accounts will pull through to both the Buffer Browser Extension and the Mobile App.
Whenever you open the composer for the first time after logging into Buffer, your default social accounts will be selected automatically. However, if you then move away to a different social account (by selecting it on the left hand side of your dashboard), the defaults will be ignored and only that social account will be selected.