Clarify need for Commitment Input for time

tips/TIP-0038/tip-0038.md

@@ -496,7 +496,7 @@ expiration.
 ### Timelock Unlock Condition
 
 Slot indices in the Tangle are introduced via slot commitments and represent a notion of time. Each such commitment
-carries an index. When using any feature related to time, a
+carries an index. When consuming a Timelock Unlock Condition, a
 [_Commitment Input_](https://github.com/iotaledger/tips/blob/tip45/tips/TIP-0045/tip-0045.md#commitment-input) is needed
 as a reference of time.
 
@@ -553,7 +553,9 @@ The expiration feature can be viewed as an opt-in receive feature, because the r
 funds after the output expires, while the return address specified by the sender regains control over them. This feature
 is a big help for on-chain smart contract requests. Those that have expiration set and are sent to dormant smart
 contract chains can be recovered by their senders, not to mention the possibility of time requests by specifying both a
-timelock and an expiration unlock condition.
+timelock and an expiration unlock condition. When consuming an Expiration Unlock Condition, a
+[_Commitment Input_](https://github.com/iotaledger/tips/blob/tip45/tips/TIP-0045/tip-0045.md#commitment-input) is needed
+as a reference of time.
 
 For the rationale behind the index choices for the slot index conditions in this Unlock Condition, refer to
 [Expiration Unlock Condition Index Bounds](#expiration-unlock-condition-index-bounds).