Query about Life cycle controller switch from internal to external clock

[DerekWangScaleflux]

Hi Caliptra team,
In the Caliptra SS integartion spec, it mentions that "Life cycle contoller allows you to switch from internal to external clock on a request from SOC over JTAG or AXI".
The query is that what's the "external clock" refering to ? Another clock other than cptra_ss_clk_i ? What's the purpose and usage of this "external clock", where it is connected to ?