The 9 variants have these members:

m1:  m1_i1   m1_i2   m1_i3   m1_p1   m1_p2   m1_p3  
m2:  m2_i1   m2_i2   m2_i3   m2_l1   m2_l2   m2_p1   m2_s1  
m3:  m3_i1   m3_i2   m3_p1   m3_ca1 
m4:  m4_l1   m4_l2   m4_l3   m4_l4   m4_l5  
m5:  m5_c1   m5_c2   m5_i1   m5_i2   m5_l1   m5_l2   m5_l3  
m6:  m6_l1   m6_l2   m6_l3   m6_s1   m6_s2   m6_s3   m6_c1   m6_c2   m6_p1   m6_p2  
m7:  m7_i1   m7_i2   m7_i3   m7_i4   m7_p1   m7_p2  
m8:  m8_i1   m8_i2   m8_p1   m8_p2   m8_p3   m8_p4  
m9:  m9_l1   m9_l2   m9_l3   m9_l4   m9_l5   m9_s1   m9_s2   m9_s3   m9_c1   m9_c2  

where the type of each member is indicated by the letter(s) following the dash _:

m?_letter?	type
c	char
s	short
i	int
l	long
p	char *
ca	char [14]

Message structures are defined in the header file

    /usr/src/include/minix/ipc.h

which also contains the #define's to shorten the names for accessing the members of the different structs in the message union.