REST API to apply a retention Label

So, if we wrote some code to apply a retention label to files in SP using the REST API, the retention label would have been created 'automatically'. Does this mean that we would need an E5 licence? (in my opinion, the word 'automatically' is extremely ambiguous in this critical context). My initial (and I believe logical) reaction is that we would NOT need an E5 licence just because we chose to develop something using the REST API. The reason I say this, is that if we did need an E5 licence, then we'd barely need the REST API, because E5 would have brought with it the capability to automatically apply a retention label. Ergo, the very existence of the REST API suggests to me that we can use it within an E3 environment. Any views?