OpenPLC uses a particular format for binding variables to addresses. It took a bit of digging around, but I’ve finally figured out what they mean and how OpenPLC interprets the addresses. The first part is to define a general identification for types:

Data Width Input Output Memory
Bit IX QX MX
Byte IB QB MB
Word IW QW MW
Double word ID QD MD
Long word IL QL ML

Items with a strikethrough are not currently supported. There is another important note. These values should indicate the data width (not the type). However, as far as I can tell, OpenPLC assumes data types based on the definitions, which means you don’t have freedom to use they as containers.

(I’m not 100% sure that my understanding of IEC 61131-3 is correct since I only have a book rather than the original specification).