Function next_code_at 

pub fn next_code_at(
    secret_b32: &str,
    params: &Params,
    unix_secs: u64,
) -> Result<String, String>

The code that replaces the current one.

For a time-based seed that is the next step; for an Hotp seed it is the next counter, which is the value the service will expect after this one is spent. Both are the same operation, which is the point.