Where are defined the man pages grouping descriptions?

The header in rpc(3) is defined by .Dt title macro:

$ zcat  $(man -w 3 rpc) | head
." @(#)rpc.3n 1.31 93/08/31 SMI; from SVr4
." Copyright 1989 AT&T
.Dd May 7, 1993
.Dt RPC 3
.Os
.Sh NAME
.Nm rpc
.Nd library routines for remote procedure calls
.Sh SYNOPSIS
.In rpc/rpc.h

You can read about it in man 7 groff_mdoc:

TITLE MACROS
     The title macros are part of the page structure domain but are presented
     first and separately for someone who wishes to start writing a man page
     yesterday.  Three header macros designate the document title or manual
     page title, the operating system, and the date of authorship.  These
     macros are called once at the very beginning of the document and are used
     to construct headers and footers only.

 .Dt [⟨document title⟩] [⟨section number⟩] [⟨volume⟩]
         The document title is the subject of the man page and must be in
         CAPITALS due to troff limitations.  If omitted, ‘UNTITLED’ is
         used.  The section number may be a number in the range 1, ..., 9
         or ‘unass’, ‘draft’, or ‘paper’.  If it is specified, and no vol‐
         ume name is given, a default volume name is used.

         Under BSD, the following sections are defined:

         1   BSD General Commands Manual
         2   BSD System Calls Manual
         3   BSD Library Functions Manual
         4   BSD Kernel Interfaces Manual
         5   BSD File Formats Manual
         6   BSD Games Manual
         7   BSD Miscellaneous Information Manual
         8   BSD System Manager's Manual
         9   BSD Kernel Developer's Manual

For example, if you changed it to .Dt RPC 9 you'd get:

RPC(9)   BSD Kernel Developer's Manual   RPC(9)