In the examples, concent \ communications \ subscriptionTypeId has a value of 999, but I’m not seeing / understanding where that 999 is coming from.
Put another way, if I created my own subscription type, how would I know its subscriptionTypeId?
You can see all available subscription types by hitting this endpoint
https://api.hubapi.com/communication-preferences/v3/definitions
An example is on this doc
@dennisedson - Great thanks.
btw, aside from via the API, when viewing a Contact > Communication subscription > Right sidebar slide out has a select. Something like this (below). You can get the ID with Inspect as well 
<span class=“private-typeahead-result private-typeahead-result–selectable” data-option-value=“THE ID IS HERE” role=“option”><span class=“”> <span class=“private-dropdown__item__label” data-option-text=“true”>Marketing Email | Marketing Information</span> </span></span>
Just btw there is a wayyyyyyy easier solution then all of the above ways mentioned which takes only a couple of seconds!
If you go to:
Settings → Properties → Contact properties → type: opted out of
then navigate to the subscription Type you want to see the id for and then go to the internal name at the end the subscriptionTypeid is shown there.
Your welcome 
Thank you!