NSO question to a ConfD related forum and not exactly an abundance of information… You probably want to ask such questions on the NSO Developer Hub.
I guess you are referning to the IETF draft https://tools.ietf.org/html/draft-ietf-isis-yang-isis-cfg-24 ?
If so, just take a look at why the
must "count(area-address) > 0" {
error-message
"At least one area-address must be configured.";
description
"Enforce configuration of at least one area.";
}
trigger.