KeyMan sees the world as a hierarchy of domains, rather like DNS domains. Each domain is contained within its superdomain, so trust in a domain is automatically trust in all its subdomains. Of course, there's no requirement that any organisation actually use subdomains (with the proviso that every domain is a subdomain of the root domain, ".").
KeyMan also believes in a level of trust, which is simply a number between 0 and 1. 0 means no trust at all, and 1 means complete trust. What exactly numbers between 0 and 1 mean is a matter of debate, but suffice it to say that they indicate varying degrees of trust.
KeyMan operates on signatures. Each signature includes two or three components. A trust level and a domain and, if the signature is of a key, a trust depth. The trust level and domain have been explained above. The trust depth is simple to understand - it is a measure of how far the trust in the key extends. So, a trust depth of 1 indicates that the signer trusts things directly signed by that key. A trust depth of 2 indicates the signer trusts things signed by that key, and anything signed by a key it has signed. A trust depth of inf (infinity) indicates trust to any depth, of course.
In each KeyMan installation, there is one special key - the root key. This belongs to the owner of the installation, and all trust springs from it. That is, to calculate the trust in a particular object, KeyMan looks at all the signature paths from the root key to the object, and calculates the path trust - and the final trust is the maximum of all the path trusts.
The path trust is calculated by simply checking that the domain of the final object is a (nonstrict) subdomain of all the signatures on the path to that object. If it isn't, then the path trust is zero. If it is, then the path depth must be at least 1 for the first signature (starting at the object end), 2 for the second, and so on. If not, the path trust is zero. If both the domain and depth checks are OK, then the trust is simply all the trust levels on the path multiplied together.
And that's it.