The doc text at the top says:
The criteria used for searching updates. The default criteria is "BrowseOnly=0 and IsAssigned=1 and IsHidden=0 and IsInstalled=0 and Type='Software'" which is effectively just critical updates.
But the param block shows the default doesn't include (AFAICT) the BrowseOnly=0 and IsAssigned=1 parts.
[string]$criteria="IsHidden=0 and IsInstalled=0 and Type='Software'"
I don't know enough about the criteria to say whether this is different or just my lack of knowledge about BrowseOnly=0 and IsAssigned=1 :)