Add rationale description

tip-0100.md

@@ -1,5 +1,6 @@
 ```
 TIP: 0100
+Type: Informational
 Layer: Application
 Title: Soulbound Tokens
 Status: Draft
@@ -95,4 +96,12 @@ Then sends the generated digital signature to the verifier.
 `SBT owner public key` in the redeem script received in messages `m`.
 Once you pass the verification, you are verified as the owner of the SBT.
 
-Note: During signing and signature verification, the message must be SHA-256 hashed and used as the signature digest.
+Note: During signing and signature verification, the message must be SHA-256 hashed and used as the signature digest.
+
+## Rationale
+
+A simple way to express SBT is to combine OP_RETURN and public key.
+In this case, it satisfies the feature that the owner of the public key can prove ownership of the SBT by providing a valid signature.
+However, it cannot handle use cases where SBT is disabled due to some factor (lost or stolen private key, etc.).
+In order to handle such cases, this specification supports revocation processing by issuer.
+However, there is a trade-off of growing the UTXO set compared to OP_RETURN.