Plain-language answer: A T-network has two series reactive elements with a shunt element between them and is common in wide-range manual tuners.
Why it changes the result
Several control settings may produce the same SWR with very different loss and component stress.
What to verify
Use the manufacturer sequence, tune at low power, minimize unnecessary reactance where instructed, and watch for heating.
The common mistake
Chasing 1:1 without considering loss or circulating current.
Complete-system boundary
Match the exact radiator, feed system, return path, frequency, height, orientation, and environment. Connector or thread fit does not establish electrical compatibility, structural capacity, or safe operation.
