According to this site (no idea how credible) it is "Relying Party X".
Context - "...RPX takes advantage of this and provides a middleman for the user depending on which service they selected they would like to use to login. The name means Relying Party X"
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此常见问题解答说：
This FAQ says:
根据此网站（没有想法有多可信）它是“依赖方 X”。
上下文 - “...RPX 利用这一点，根据用户选择的服务来登录，为用户提供中间人。该名称的意思是依赖方 X”
According to this site (no idea how credible) it is "Relying Party X".
Context - "...RPX takes advantage of this and provides a middleman for the user depending on which service they selected they would like to use to login. The name means Relying Party X"