diff --git a/README.md b/README.md
index 74f7043..e82bc75 100644
--- a/README.md
+++ b/README.md
@@ -188,5 +188,12 @@ You may use some instant adhesive to fix stickers.
   * "update" reboot into firmware update mode.
 * Given my limited hobby time, the firmware may not be fully tested. Please report any anomalies.
 
+## Card ID Logic
+To support all kinds of different NFC cards or tags, card IDs are transformed with the following rules (apply to both AIME and CardIO).
+* 15693 => Original 8-byte UID
+* MIFARE (4-byte UID) => 0xE0 + UID + first 2 bytes of the UID
+* MIFARE (7-byte UID) => 0xE0 + UID
+* FeliCa => Original 8-byte IDm
+
 ## 3D Model Source File (Onshape)
 https://cad.onshape.com/documents/ca5497f91b2962105335e822/w/7b88022e98c02c60ad0c44a7/e/c3476efd13c08f807f3773fe?configuration=List_6ARRO0azcgmmHg%3D__&renderMode=1&rightPanel=configPanel&uiState=6558cabf9b380560ca5b554e