[FROG] What's this 'interface' mean in 'neighbor' configure node ?

Hi all, I found this is new in FRR compared to QUAGGA, what's this used for ? Why I should config interface first ? Is there some DOC about this ? # neighbor 1.1.1.1/24

... interface Enable BGP on interface

# neighbor 1.1.1.1/24 remote-as 5001

% Create the peer-group or interface first or specify "interface" keyword % if using an unnumbered interface neighbor

---- Simon Jones