不明白结果中的PER_EFFECT 和@ISPERIOD是什么意思？谢谢！

运行结果如下：

LS(CX=F,PER=F,COV=CXWHITE) Y2? X1? X2?

Estimation Equations:
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PER_EFFECT = C(41)*@ISPERIOD("1999") + C(42)*@ISPERIOD("2000") + C(43)*@ISPERIOD("2001") + C(44)*@ISPERIOD("2002") + C(45)*@ISPERIOD("2003") + C(46)*@ISPERIOD("2004")

Y2B06 = C(4) + PER_EFFECT + C(1) + C(2)*X1B06 + C(3)*X2B06

Y2B07 = C(5) + PER_EFFECT + C(1) + C(2)*X1B07 + C(3)*X2B07

Y2B08 = C(6) + PER_EFFECT + C(1) + C(2)*X1B08 + C(3)*X2B08

Y2B09 = C(7) + PER_EFFECT + C(1) + C(2)*X1B09 + C(3)*X2B09

Y2B10 = C(8) + PER_EFFECT + C(1) + C(2)*X1B10 + C(3)*X2B10

Y2C13 = C(9) + PER_EFFECT + C(1) + C(2)*X1C13 + C(3)*X2C13

Y2C14 = C(10) + PER_EFFECT + C(1) + C(2)*X1C14 + C(3)*X2C14
。。。。