Part of his salary is designated as a travel allowance for work-related trips.

allowance 是名词，来自动词 allow（允许、准许），加上名词后缀 -ance 构成，表示"允许给予的东西"或"津贴"。 travel allowance 指为公务出差提供的"差旅津贴"，用于报销因公出行产生的交通、住宿等费用。