As the specification states, there are two classes of fields. These two classes are those fields containing a single value, and those fields containing multiple values.
A single value field may contain a single number, a single vector, or a single image. The names of these fields always begin with the letters "SF."
A multiple value field contains multiple values. The names of these fields always begin with the letters "MF." These fields are written as a series of values separated by commas, all of which are enclosed by brackets. If a particular field has no values in it then only the brackets are written. If the field only contains one value, and one value exactly, you can forget about using the brackets and just write the value. The last value in a multiple value field can be followed by a comma but does not have to be. For example, all of the following code fragments are valid representations of a multiple value field containing a single integer value of 1.
1
[1,]
[1]