The split-GAL4 system allows you to restrict expression from a UAS construct to cells where the two components of the split-GAL4 activator are expressed. As shown below, the GAL4 DNA-binding domain fused to the Zip- protein-pairing domain can be expressed in one pattern, a transcriptional activation domain fused to the Zip+ protein-pairing domain can be expressed in another pattern and the UAS reporter construct will be expressed in the intersection of the two patterns.
As also shown below, this system can be extended to reflect three expression patterns by including a construct expressing a protein that competes for Zip- binding. This "Killer Zipper" protein, consisting of the GAL4 DNA-binding domain fused to the Zip+ protein-pairing domain, can be expressed directly under control of cell-specific regulatory sequences or indirectly under LexAop control.
An example is shown below of Killer Zipper expression eliminating split-GAL4 activation of UAS-Venus expression in the nerve cord, but not the brain lobes.