Comoving absorption path length (ΔX) as a function of H I column density sensitivity. The results for the spin temperature to source covering fraction ratios of 100 and 1000 K are depicted as solid and dashed lines, respectively. The comoving absorption paths spanned by our data are labelled in blue (intervening absorption) and red (associated absorption). The predicted results from H I absorption searching in map-making data from the full coverage of the FAST observable sky are presented in green (intervening absorption) and orange (associated absorption).