What is an "alive indicator?" (GB Printer)

There isn't any documentation on the "Alive indicator" byte of the GB Printer packets.
The only mention is that it... _exists_, because it shows up in the table of a Printer packet.

[Edit] Also, what is a 'line' defined as?

Start with the GB Printer packet table referenced in the issue and trace the surrounding description of its bytes. Document what the “Alive indicator” byte represents and define what a “line” means in this context; done means both terms are explained in the relevant reference text.

Written by the indexing model from the issue text.