Backend developer
Responsible for what you do not see: orders, users, stock, connections to external systems, data security. On a shop this is half the project.
When your project needs this person
On a shop, a booking system, anything that stores customer data.
What happens when they are missing
Data leaks or is lost. The most common version is orders that go nowhere, because a connection to another system broke silently and nobody set up an alert.
How this appears in an offer
Appears as "integrations" or as development hours. Every external connection is separate work and should be a separate line.
What they do NOT do
Not responsible for how the site looks.