Gforth has a data stack (aka parameter stack) for characters, cells, addresses, and double cells, a floating point stack for floating point numbers, a return stack for storing the return addresses of colon definitions and other data, and a locals stack for storing local variables. Note that while every sane Forth has a separate floating point stack, this is not strictly required; an ANS Forth system could theoretically keep floating point numbers on the data stack. As an additional difficulty, you don't know how many cells a floating point number takes. It is reportedly possible to write words in a way that they work also for a unified stack model, but we do not recommend trying it. Instead, just say that your program has an environmental dependency on a separate FP stack.
Also, a Forth system is allowed to keep the local variables on the return stack. This is reasonable, as local variables usually eliminate the need to use the return stack explicitly. So, if you want to produce a standard complying program and if you are using local variables in a word, forget about return stack manipulations in that word (see the standard document for the exact rules).
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