What is the value of CLUSTER in Masscomp RTU?

[Augustine Cano]

The constant CLUSTER is used repeatedly by the code present in "Writing a Unix
Device Driver" by Egan and Teixeira.  However, I haven't been able to find
any #define where it is acually defined.  What is a CLUSTER in RTU anyway?
What is the value of this constant?

Thanks in advance.  I'll summarize e-mail responses; the resulting article
is bound to be quite minimal...

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