帮忙给看一段程序

FINISH
/CLEAR
/PREP7
B=10
H=20
L1=100
L2=150
A1=B*H
I1=B*H**3/12
I2=H*B**3/12
ET,1,SOLID95
ET,2,BEAM4
MP,EX,1,3E5
MP,PRXY,1,0.3
R,1,A1,I1,I2,B,H
BLC5,,,B,H,L1
k,99,,,100
K,100,,,L1+L2
L,99,100
LSEL,S,LOC,Z,L1+1,L1+L2
LATT,1,1,2
LESIZE,ALL,,,10
LMESH,ALL
VATT,1,,1
ESIZE,2.5
MSHAPE,0
MSHKEY,1
LSEL,S,LOC,Z,1,L1-1
LESIZE,ALL,B/2
VMESH,ALL
ASEL,S,LOC,Z,0
DA,ALL,ALL
FK,100,FX,100
FK,100,FY,-200
NSEL,S,LOC,Z,L1
CERIG,1,ALL
ALLSEL,ALL
/SOLU
SOLVE
这是我在书上看到的，有点疑问
LSEL,S,LOC,Z,L1+1,L1+L2，为什么要出现个加1减一呢，应该是LSEL,S,LOC,Z,L1,L1+L2啊，还有 CERIG,1,ALL，这些1是怎么出来的啊