Can someone tell me the difference between a form dies and a form & trim die?

[briannmilewis]

Web search is not at all helpful.

[Ranch13]

 A form die is sometimes used when making cases from another "parent" case. Making 577/450 martini's from 28 ga brass shotshells etc.
 A trim die is to run a case up into and then either file or use a hacksaw to trim the case to the proper length.

[.22-5-40]

Hello, briannmilewis.  And sometimes they can be one and the same!..I have a 3 piece RCBS form die set for making .219 Donaldson Wasp from .30-30 (darn set cost more than the rifle!)  They are numbered 1 thru 3.
   The 3rd. die completes the necking down/forming process & also serves as trim die.

[anachronism]

Trim dies are made to a specific case length, and are hardened on the top so you can file the case down to length. Not all forming dies are finished in such a manner.

[wncchester]

Form dies are made to step down cases from large to small in changes the brass can withstand without collapsing.  Some are a singles; like .30-06 to .280.  Some will be as many as five; like .30-06 to .22-250.  The last die in any of them will be a trim die that is basically just a FL die cut to proper length and without a decapping assembly.
 
It appears stand alone trim dies have been dropped due to a lack of consumer demand.  That's unfortunate because they are excellant tools as well as quick and easy 'trimmers'.