at arm's Customers and suppliers must be dealt with fairly a

at arm's length 指保持一点距离,避免太亲近或太熟悉.
例句：
Bill hated seeing his colleagues outside the office,preferring to keep all of them at arm's length
She was friendly only when he was safely at arm's length.
问题中句子的意思是供应商和客户之间的关系要公平合理,同时也不要太接近.
因为过于接近和亲近会影响进行公正的判断和公平的交易.