This feature greatly speed up input of commonly used sentences, or text that will be use in your messaging content. The Text Replacement list can be found in Email editor and custom SMS message pages.
You could just click on the replacement option you need to use from the list.
List can be found in Email Template Editor> Create/edit a Text> Merge tags>
List can seen in SMS Activity Messaging> Select a activity to edit> select custom>
For quick references, you may view the below list. More option are available on the individual custom messaging page.