Okuma Mill Program

Can someone explain NAT18 (3/4" INSERT MILL .7500) We use it to open up different size holes 1,1 1/2,1 3/4 and so on.When it calls up the sub OMIL1 what does the I mean.I control the size with the D18 cutter comp I think I would like to change just the I for different size holes.

N0001 (12" X 12" ALUMINUM PLATE)

N0011 G00 G90 Z20.
N0012 /X-10. Y20.
N0013 M01
NAT02 (3/4" X 118 DEG. SPOT DRILL)
N0200 M6 T02
N0201 G15 H01
N0202 G56 H02
N0203 M03 S2000
G00 X0 Y0
Z.1 M08
G1 Z-.12 F5.
G0 Z3.5
N0204 G71 Z3.5
N0205 CALL OSPT1
N0206 G00 G80 Z20. M09
N0207 /X-10. Y20. M05
N0208 M01
NAT06 (13/32" DIA. DRILL .4062)
N0600 M6 T06
N0601 G15 H01
N0602 G56 H06
N0603 M03 S1625
N0604 G71 Z3.5 M08
N0605 CALL ODRL1
N0606 G00 G80 Z20. M09
N0607 /X-10. Y20. M05
N0608 M01
NAT07 (9/16" DIA. DRILL .5625)
N0700 M6 T07
N0701 G15 H01
N0702 G56 H07
N0703 M03 S1475
N0704 G71 Z3.5 M08
N0705 CALL ODRL2
N0706 G00 G80 Z20. M09
N0707 /X-10. Y20. M05
N0708 M01
/NAT18 (3/4" DIA. INSERT MILL .7500)
/N1800 M6 T18
/N1801 G15 H01
/N1802 G56 H18
/N1803 M03 S2500
/N1804 G00 X0 Y0
/N1805 Z.1 M08
/N1806 G01 Z0 F18.
/N1807 CALL OMIL1
/N1808 G00 Z20. M09
/N1809 /X-10. Y20. M05
/N1810 M01
/NAT01 (1/2" X 118 DEG. SPOT DRILL)
/(INSERT .134 FOR TOOL COMP IN D01)
/N0100 M6 T01
/N0101 G15 H01
/N0102 G56 H01
/N0103 M03 S3000
/N0104 G00 X0 Y0
/N0105 Z.1 M08
/N0106 G01 Z-.088 F35.
/N0107 G41 X.5 D01
/N0108 G03 I-.5
/N0109 I-.5 F70.
/N0110 G01 G40 X0 F100.
N0111 G00 Z20. M09
N0112 X-10. Y20. M05
N0113 M19
N0114 M30
(********** SUB PROGRAMS *********)
OSPT1 (3/4" X 118 DEG. SPOT DRILL)
NCYL G81 Z-.11 R.1 P.25 F5. M53
CALL OLOC1
NCYL G81 Z-.15 R.1 P.25 F5. M53
CALL OLOC2
RTS
(*********************************)
ODRL1 (13/32" DIA. DRILL .4062)
NCYL G81 Z-.75 R.15 F9. M54
CALL OLOC1
RTS
(*********************************)
ODRL2 (9/16" DIA. DRILL .5625)
NCYL G81 Z-.75 R.1 F9. M53
CALL OLOC2
RTS
(*********************************)
OMIL1 (3/4" DIA. INSERT MILL .7500)
G41 X.5 D18
G03 I-.5 Z-.05
I-.5 Z-.1
I-.5 Z-.15
I-.5 Z-.2
I-.5 Z-.25
I-.5 Z-.3
I-.5 Z-.35
I-.5 Z-.4
I-.5 Z-.45
I-.5 Z-.5
I-.5 Z-.55
I-.5 Z-.6
I-.5 F36.
G01 G40 X0 F100.
RTS
(*********************************)
OLOC1 (13/32" DIA. HOLE LOCATIONS)
BHC X0 Y0 I4.500 J-45. K4
RTS
(*********************************)
OLOC2 (9/16" DIA. HOLE LOCATIONS)
BHC X0 Y0 I7.0710 J-45. K4
RTS
(********** END OF SUBS **********)