请问大家这段代码是什么意思啊？

function [sys,x0,str,ts] = tubemodelBandigital(t,,,flag) switch flag case 0 sizes = simsizes; sizes.NumContStates = 0; sizes.NumDiscStates = 0; sizes.NumOutputs = 0; sizes.NumInputs = 0; sizes.DirFeedthrough = 1; sizes.NumSampleTimes = 1; sys = simsizes(sizes); x0=[]; str=[]; ts=[0,0]; set_param('BanDigitalModel/Flowtube/H(s)','Denominator','[1,2pi11620.002,4pi^2116^2]'); case 3 if(t<3||t>=18) set_param('BanDigitalModel/Flowtube/H(s)','Denominator','[1,2pi11620.002,4pi^2116^2]'); sys=[]; end if((t>=3&&t<6)||(t>=15&&t<18)) set_param('BanDigitalModel/Flowtube/H(s)','Denominator','[1,2pi20320.0005,4pi^2*203^2]'); sys=[]; end if((t>=6&&t<9)||(t>=12&&t<15)) set_param('BanDigitalModel/Flowtube/H(s)','Denominator','[1,2*pi*203*2*0.007,4*pi^2*203^2]'); sys=[]; end if(t>=9&&t<12) set_param('BanDigitalModel/Flowtube/H(s)','Denominator','[1,2pi20320.03,4pi^2*203^2]'); sys=[]; end case {1,2,4,9} sys = [];end