The underlying concept is called Visible type application and allows to provide explicit type arguments to a function. This was discussed earlier in this forum here: I've just discovered what the `@<template_name>` notation in functions like `fetchByKey` et al means
In this particular case, the function query comes with two type parameters t and p, which can be specified with the @-notion and correspond to the order defined by the function. You could technically write query @BankAccount @Party bank, resulting in the same, although that is not necessary since only t is ambiguous when not defined.