The legislation establishes the identity of the principal
company by looking upwards from Company A (the lessor company) and
establishing whether Company A is a 75% subsidiary of another
company.
This principle is illustrated in the following examples.
Example 1

B Ltd is a principal company of A Ltd.
But A Ltd might not be in such a simple group relationship.
The legislation deals with more complex structures by tracing
upwards from A Ltd and identifying each 75% subsidiary relationship
so that a chain of ownership can be established. Wherever there is
a 75% subsidiary relationship between a company and another company
above it there is a link in a chain of ownership. When you reach a
company that is not a 75% subsidiary of another company no higher
links in the chain can be found, the chain stops and the top
company is the principal company.
Example 2

A Ltd is a 100% subsidiary of B Ltd but B Ltd is a 100%
subsidiary of C Ltd. C Ltd is not a 75% subsidiary of another
company so C Ltd is the principal company of A Ltd.
Example 3
The test looks at one link of the chain at a time so that in
this example:

A Ltd is a 75% subsidiary of B Ltd. B Ltd is a 75% subsidiary of C Ltd. C Ltd is not a 75% subsidiary of another company so C Ltd is the principal company of A Ltd. It does not matter that A Ltd is not a 75% subsidiary of C Ltd as defined [ in [xxxx] for the purpose of group relief.]