[ENH] BEP001 - New entity: flip

src/99-appendices/09-entities.md

@@ -133,13 +133,26 @@ Full name: Echo
 
 Format: `echo-<index>`
 
-Definition: Multi-echo data MUST be split into one file per echo.
-Each file shares the same name with the exception of the `_echo-<index>`
-key/value.
+Definition: If constituents of an entity-linked file collection differ as a
+function of `EchoTime` acquisition parameter, the `_echo-<index>` key/value
+pair MUST be used to distinguish individual files.
 Please note that the `<index>` denotes the number/index (in the form of a
 nonnegative integer) of the echo not the echo time value which needs to be
 stored in the field `EchoTime` of the separate JSON file.
 
+## flip 
+
+Full name: Flip Angle
+
+Format: `flip-<index>`
+
+Definition: If constituents of an entity-linked file collection differ as a
+function of `FlipAngle` acquisition parameter, the `_flip-<index>` key/value
+pair MUST be used to distinguish individual files.
+Please note that the `<index>` denotes the number/index (in the form of a
+nonnegative integer), not the `FlipAngle` value which needs to be
+stored in the field `FlipAngle` of the separate JSON file.
+
 ## recording
 
 Full name: Recording