First off, you're highly unlikely to hit your bandwidth limit on DigitalOcean even if you are on the lowest of the low end droplet with no other bandwidth-boosting services.
That said, there are four types of traffic on DigitalOcean when it comes to droplets: outbound local, inbound local, outbound remote, and inbound remote. Any transaction will do some of both, but likely will be skewed in the direction of one or the other.
Case in point: I have a droplet that runs a Neo4j database server, and another droplet that queries that server with Neo4j (graph database) drivers. When my client server makes a request to the database, it sends a database query through the driver, which is a small outbound remote from the client POV (the servers are in separate projects, so it's not "internal traffic" like if they were using internal IPs in the same project) and a small inbound remote for the database server. The actual substantive data transfer is the query response, which in the case of my workloads tends to be a few hundred MB at minimum, which shows up as remote outbound for the database server and remote inbound for the client.
To sum up: any transfer outside of DigitalOcean resources in the same virtual network counts as outgoing when you send data (ev like curl or wget) and a likely much larger inbound when you receive it. Vice versa if you're responding to requests with a lot of data.
