Trivial I would reverse the order of the paragraphs - 1743 and 1747. The flow follow the order of the process.
1747 The sender decides whether to use an alias and chooses the alias value. It establishes an alias by
1748 including a non-zero length Topic Name and a Topic Alias value in the PUBLISH packet. When it receives
1749 a packet like this the receiver processes the packet as normal, but it also records the mapping of Topic
1750 Alias to full Topic Name.
The sender establishes an alias by including a non-zero length Topic Name and a Topic Alias value in the PUBLISH packet. When it receives such a packet, a receiver processes it as normal and also records the mapping of Topic Alias to its corresponding Topic Name.
1743 If a Topic Alias has been established at the receiver a sender is permitted to send a PUBLISH packet that
1744 contains that Topic Alias and supplying a zero-length String in place of the Topic Name.
Once a Topic Alias has been established at the receiver[,], a sender is permitted to send a PUBLISH packet that contains that Topic Alias and a zero-length String in place of the Topic Name.