I/O scheduling, iotune and difference between read_bytes_sec and read_bytes_sec_max in qemu/kvm

Issue

I want to throttle the I/O using iotune and I wondering what is the difference between read_bytes_sec and read_bytes_sec_max? https://libvirt.org/formatdomain.html describes these tunables as:

read_bytes_sec
    The optional read_bytes_sec element is the read throughput limit in bytes per second.

and

read_bytes_sec_max
    The optional read_bytes_sec_max element is the maximum read throughput limit in bytes per second.

I understand it like read_bytes_sec is soft limit (can be breached when there are enough resources) and read_bytes_sec_max is hard limit which can never be breached. Is this assumption correct? What is the mechanism of evaluation if the disk is overloaded? Can the read_bytes_sec be trusted?