Buprenorphine is unique as a treatment medication.
An agonist is a drug that activates certain receptors in the brain. Full agonist opioids activate the opioid receptors in the brain fully resulting in the full opioid effect. Examples of full agonists are heroin, oxycodone, methadone, hydrocodone, morphine, opium and others.
An antagonist is a drug that blocks opioids by attaching to the opioid receptors without activating them. Antagonists cause no opioid effect and block full agonist opioids. Examples are naltrexone and naloxone. Naloxone is sometimes used to reverse a heroin overdose.
Buprenorphine is a partial agonist meaning, it activates the opioid receptors in the brain, but to a much lesser degree than a full agonist.
Buprenorphine also acts as an antagonist, meaning it blocks other opioids, while allowing for some opioid effect of its own to suppress withdrawal symptoms and cravings.
This is why it would be misleading to classify buprenorphine as a replacement therapy. It would be equally misleading to classify it solely as an opioid blocker. Buprenorphine is in a category of its own and therefore should not be seen as “replacement” or "substitution" for anything else.