It determines whether a component is fixed or floating.
I am not sure what you searched for in the help, but searching for $fixed like this:
Gives this help topic as the second search result:
This help topic, which is the 9th topic in the search results gives a summary of all of the different design table parameters:
I hope this helps,
Go figure... The $ in $FIXED doesn't work/help when used in a Search string in SW Help nor Google.