PE04 create function ,development class choose what?

hi,

when I create a customer function (tcode:PE04), name ZLFCU,  when I save it, I choose Development class of Z001(the customer difine),but the system give this message:

"SAP object PFCS ZLFCU cannot be assigned to development class Z001

Message no. TO 142

Diagnosis

Object PFCS ZLFCU is in the name range reserved for SAP. The object cannot be assigned to development class Z001, since it belongs to a different name range or namespace.

"

I don't know why?  but if  I choose "PCAL" ,it's permited.

so who can help me, thanks.

Olivia