You are looking for public sculpture that is of abstract form, or an abstracted representation of a form (for example, a giant hand made of thin twisted wire). There are many definitions of Abstract art, but no definitive one. A few lines from Wikipedia states, "Abstraction indicates a departure from reality in depiction of imagery in art. This departure from accurate representation can be only slight, or it can be partial, or it can be complete. Abstraction exists along a continuum. Total abstraction bears no trace of any reference to anything recognizable. Figurative art and total abstraction are almost mutually exclusive. But figurative and representational (or realistic) art often contains partial abstraction." Wikipedia Abstract Art
The Figurative Public Sculpture category does allow for 100% representational pieces to slightly abstracted human, animal or fantasy creature forms. Abstract Public Sculptures allows for pieces that are beyond 'slightly abstracted.' Thus, there is a blurring of the lines between the categories. That is the nature of art.
The work must be located in an area that is open to the public and free of charge. A sculpture in an airport that is within the secure area (or any other secured location) would not fit this category as only people with tickets would be able to view the sculpture. The sculpture should be of a permanent nature; not a temporary exhibit that ends at a set time period. Consumer mass-market items commonly made of tin, plaster, plastic, concrete, etc., such as garden/patio sculptures are prohibited.