Hi Creator, Convert to sheet metal convert a non-sheet metal part to a valid sheet metal part so that you can perform sheet metal operations like bending, unbending, flattening, and so on. You can convert solid models and imported non-sheet metal parts to sheet metal parts. Converting solid models The solid models must have a constant thickness. You cannot convert a sheet body to a sheet metal part.The solid model is created in the Modeling application and then converted to a valid sheet metal part using the Convert to Sheet Metal command.
https://docs.plm.automation.siemens.com/tdoc/nx/11/nx_help#uid:sm_from_solid_options Better you go with this site.
Please refer NX Help Documentation for detail.
: You can select the faces from the solid to be panel (web) faces, and edges between the faces to be bend regions
Have a look at the movie below that shows these basic differences.