From VFP help:

LOOKUP(ReturnField, eSearchExpression, SearchedField, cTagName)

I cannot understand the meaning of third parameter, if fourth
parameter is present.

If I specify cTagname, eSearchExpression is searched using tag cTagname
and there is no any need to use SearchedField.

What in this case I must put to SearchedField ?

If I leave SearchedField empty, error occurs. Is this an illegal
VFP design ?