This is the apparent thermal resistance $T_{b0}$ used to calculate cable partial transient temperature rise acc. to IEC 60853-2.

Symbol
$T_{\mathrm{b0}}$
Unit
Formulae
$T_{\mathrm{A}} + T_{\mathrm{B}} - T_{\mathrm{a0}}$
Related
$T_{\mathrm{A}}$
$T_{\mathrm{a0}}$
Used in
$\alpha_{\mathrm{t}}$
$\Delta \theta_{\mathrm{c_{\mathrm{t}}}}$
$\Delta \theta_{\mathrm{d_{\mathrm{t}}}}$