Suppose the spin orbit coupling term is written below. $$H_{soc} = \xi L\cdot\sigma$$ where, $\xi$ is the strength of the spin orbit coupling, $L$ and $\sigma$ are angular momentum and pauli matrix operators respectively.
The exchange potential term is written in the following. $$m_z \times \sigma$$ where, $m_z$ is a magnitude of exchange potential and $\sigma$ is the pauli matrix
When the time reversal symmetry operator is applied onto these two terms respectively, why does the spin orbit coupling term change the sign; while, does not the exchange potential term?