Create a categorical array, A.

A = categorical({'plane' 'car' 'train' 'car' 'plane'})

A = 1x5 categorical
     plane      car      train      car      plane 

A is a 1-by-5 categorical array.

Display the categories of A.

C = categories(A)

C = 3x1 cell
    {'car'  }
    {'plane'}
    {'train'}

Since you created A by specifying only an input array, the categories appear in alphabetical order.

Create an ordinal categorical array.

A = categorical({'medium' 'large'; 'small' 'xlarge'; 'large' 'medium'},...
    {'small' 'medium' 'large' 'xlarge'},'Ordinal',true)

A = 3x2 categorical
     medium      large  
     small       xlarge 
     large       medium 

A is a 3-by-2 ordinal categorical array.

Display the categories of A.

C = categories(A)

C = 4x1 cell
    {'small' }
    {'medium'}
    {'large' }
    {'xlarge'}

The categories appear in the order in which you specified them. Since A is ordinal, the categories have the mathematical ordering small < medium < large < xlarge.