Having looked into https://linuxcontainers.org/lxc/manpages/man5/lxc.container.conf.5.html
lxc.start.order An integer used to sort the containers when auto-starting a series of containers at once. AUTOSTART AND SYSTEM BOOT ... The startup will be in order of lxc.start.order.
The leaves one to guesswork of how the integer is interpreted, e.g. whether 0 is lowest or highest priority or even whether 0 might be a special value (like perhaps disabling)
It thus requires a (potential) user to sour other sources (e.g. https://stgraber.org/2013/12/21/lxc-1-0-your-second-container/) to discover
priority of the container, higher value means starts earlier
I suppose such should be rather pointed out in the man pages.