In Botan's example code, I see various ways of creating keys and IV/nonce which include a combination of either Botan::secure_vector<uint8_t> OR std::vector<uint8_t>.

Primarily the keys are constructed using std::vector and IV is constructed using secure_vector. I would assume the keys also need to be constructed with secure_vector given what the documentation says about using secure_vector in place of std::vector.

Essentially I am asking what are the use cases of using either one of those container types (secvec vs normal vector) in our own application code.