A scalar struct is one for which size() of the struct itself says [1 1]. It does not matter what size the fields contained in the struct are, just the size of the struct itself. For example,
test1.this = 1:20;
test1.that = 'hello';
test2(1).this = 1;
test2(1).that = char('what','is','the','result','here?');
test3(1).this = 1:20;
test3(1).that = 'hello';
test3(2).this = 1;
test3(2).that = char('what','is','the','result','here?');
With these lines, size(test1) and size(test2) will both be [1 1] -- they are scalar structures, no matter how big the fields they hold are. Referring to test1.that would give you the single result 'hello' .
However, size(test3) will be [1 2]: it is not a scalar structure, it is a structure array. Referring to test3.that would give you a comma-separated list of two results, the first of which was 'hello' and the second of which was the char array, because of "structure expansion".
