Piece of Lua syntax that I don't understand

I'm using a Lua-based product, I'm using their API and there is a bit of syntax in there that I don't understand.

What is this? Is it a function call for Add, and if so what are the input parameters - there's nothing assigning that table to the variable input - no equals sign?

Is it a function definition for Add - that would seem odd, without any implementation and specifying what goes into the input tables?

Is Add a table containing tables? I've never seen a table created with parentheses instead of curly braces?

serviceDefinitions.Add(
    input { name="p1", baseType="NUMBER", description="The first addend of the 
    operation" },
    input { name="p2", baseType="NUMBER", description="The second addend of the operation" },
    output { baseType="NUMBER", description="The sum of the two parameters" },
    description { "Add two numbers" }
)