That was a topic of a thread about a year ago and the cause of considerable confusion when ti was added.  The (potential) problem is that a multiple line Sales Order could have different Estimated Ship Dates and there is no way for the SO-C to know which linr to look at.  What the current version of SO-C does is:
 
Prompt "Use Existing SO Ship Date?"  
 
If you say N then it asks you for a ship date.
If you say Y, then if you have already released the order and entered a Ship Date in SO-E, it uses that date.  If not, it pulls the Estimated Ship Date from the first line.  So if you have blanket orders, and the first line shipped months ago, you need to be really careful.