What is control-rate omxh265enc plugin

Hi Gstreamer dev team,
I'm exploring  omxh265enc plugin and saw that it has control-rate property
control-rate : Bitrate control method flags: readable, writable, changeable
only in NULL or READY state Enum "GstOMXVideoEncControlRate" Default: -1,
"default" (0): disable - Disable (1): variable - Variable (2): constant -
Constant (3): variable-skip-frames - Variable Skip Frames (4):
constant-skip-frames - Constant Skip Frames (2130706434): capped-variable -
Capped Variable (2130706433): low-latency - Low Latency

Is it something related to dynamic bitrate support ?
I saw the default is variable , does it mean it uses dynamic bitrate by
default and which criteria to decide the bitrate of omxh265enc ?

Best regards,
Hau Lam