初学者；求解释下边程序每一步

UCAS-蜡笔小新

gen edge 2.0     网格划分
change mat 1 range z 0,10
change mat 2 range z 10,15                             change mat   与 change jmat 有区别吗？   prop mat 三者的联系
change mat 3 range z 15,25
prop mat=1 dens 1600 bulk 5e9 g 3e9   
prop mat=2 dens 2000 bulk 5e9 g 3e9
prop mat=3 dens 2200 bulk 5e9 g 3e9
change jmat 1
prop jmat 1 jkn 1e10 jks 1e10 jcoh 1e10
gravity 0 0 -10
insitu stress 0.0,0.0,-4.8e5 0,0,0 &
zgrad 0.0,0.0, 2.2e4 0,0,0 range z 0,10
insitu stress 0.0,0.0,-4.6e5 0,0,0 &
zgrad 0.0,0.0, 2.0e4 0,0,0 range z 10,15
insitu stress 0.0,0.0,-4.0e5 0,0,0 &
zgrad 0.0,0.0, 1.6e4 0,0,0 range z 15,25
bound xvel = 0.0 range x 0.0                        施加速度
bound xvel = 0.0 range x 20.0
bound yvel = 0.0 range y 0.0
bound yvel = 0.0 range y 20.0
bound zvel = 0.0 range z 0.0
hist unbal
step 500
plot create plot Z-stress
plot set dip 90 dd 180
plot cut add plane origin (10,10,12.5) name Plane normal (0,1,0)                            这一步完全看不懂
plot add bcont szz minimum -6e5 maximum 0 interval 1.25e5 wireframe off