It is in the document. Check this example in the document. One creates a 1x2 struct array. The other creates a single struct.
>> s = struct('type',{'big','little'},'color','red')
s =
1×2 struct array with fields:
type
color
>> s = struct('type',{{'big','little'}},'color','red')
s =
struct with fields:
type: {'big' 'little'}
color: 'red'
