"ChapNum" is the name of the sequence; you can choose something else, if you wish. But you have to use the same name for all occurrences of the field, since the name identifies SEQ fields that are part of the same sequence.
Did you make sure NOT to type the field braces (because doing so won't work)? The
+ F9 shortcut is supposed to work in Word for Mac (but I can't test
that, because I'm running Word for Windows here).
The SEQ fields actually creates a sequence of numbers (1, 2, 3, etc.) which is then multiplied by 5 and, finally, 100 is added to the result.
The SEQ field is documented in the support article at https://support.microsoft.com/en-us/office/field-codes-seq-sequence-field-062a387b-dfc9-4ef8-8235-29ee113d59be?ui=en-US&rs=en-US&ad=US.
In my example, I have omitted the very first number (100), but you can just as type it as a static number (directly from the keyboard). You can find my example in the shared folder at
https://1drv.ms/u/s!AvS-HIVsUYvxh02-XNKCMfJ6Gjyr?e=0N4zG7.