HAL/S is written in a dialect of PL/I known as XPL
The three key factors in writing the language were reliability, efficiency, and machine-independence.
The language is designed to allow tasks such as performing aerospace related vector/matrix arithmetic to be accomplished in a way that is easily understandable to people who have studied the subject.
HAL/S is written without functions (such as GOTO in BASIC) that are known to be the cause of many errors. There are no abreviations for keywords, and keywords are all reserved so that they cannot also be used as variables. Considerations such as this are designed to reduce the chances of errors occurring, and also make it easy for others to read and understand the programs produced.
HAL officially stands for High-order Assembly Language, though the fictional HAL 9000 computer featured in 2001: A Space Odyssey may well have been an inspiration, as could have the German word Hals.