The /etc/pve/lxc/<CTID>.conf files stores container configuration, where "CTID" is the numeric ID of the given container. Note that IDs ⇐ 100 are reserved for internal purposes.
The file uses a simple colon separated key/value format. Each line has the following format:
OPTION: value
Blank lines in the file are ignored, and lines starting with a # character are treated as comments and are also ignored.
One can use the pct command to generate and modify those files.
It is also possible to add low-level lxc style configuration directly, for example:
lxc.init_cmd: /sbin/my_own_init
or
lxc.init_cmd = /sbin/my_own_init
Those settings are directly passed to the lxc low-level tools.