设计说明英文字母

System design manual. In technical writing you use the word "manual" for 说明书, not "guide". "instruction" and "specification" are not 说明书.

System design specification

explanation of design

Design Explanation 或Design Descriptionexplanation带有解释、说明的意思description带有描述、说明的意思两种表达都可以，请相信我的答复。