What is the “system sequence” field used for in Visio?

Question

When placing message calls (arrows from one active lifeline to another) in a system sequence diagram in Visio, I noticed that there is a property called "system sequence." Anyone know what the intended usage of this field is?

Answer 1

SOrry this question was asked way too fast I found the answer in the Visio's built in help:

Type a sequence expression for a message. In sequence diagrams, you can omit sequence numbers. In collaboration diagrams, message sequence numbers are usually nested to indicate levels. For example, message 3.1.4 follows message 3.1.3 within activation 3.1.