The discussion focuses on utilizing HEX cards in MCNP input files, with an emphasis on understanding their definition and application. A primer on MCNP is provided, which includes a link to a detailed guide that explains the use of macrobodies, specifically using RCC as an example. Additionally, tutorials are recommended to assist users in writing input files. A specific challenge arises regarding the fill card, where confusion exists about whether to format it as a matrix or in rows. The suggestion is made to search for examples from other users to clarify the correct approach.