5438UCS模块疑问

南征北战

5438的时钟，在产生错误的时候文档上是这么说的：
If a fault is detected for the oscillator sourcing MCLK , MCLK is automatically switched to the DCO for its clock source(DCOCLKDIV) for all clock sources except XT1 LF mode. If MCLK is sourced from XT1 in LF mode , an oscillator fault causes MCLK to be automatically switched to the REFO for its clock source(REFOCLK).This does not change the SELM bit settings. This condition must be handled by user software.

时钟会自动切换，但是又说要在用户程序中操作SELM。这不是矛盾了吗？应该是什么意思呢？
谢谢！