Technically, LH will assist in a subject that the HOD does teach (e.g. LH/Maths might split the levels with the HOD/Maths). Whereas SH is pretty much the IC of a subject within the department that the HOD doesn't teach (e.g. SH/Biology if the HOD/Science is not Bio-trained).

In practice, I've observed that schools may not really care about the distinction… e.g. I've seen plenty of SH/EL or SH/Maths (which doesn't really make sense). But anyway, they are effectively equivalent in terms of 'rank' lah.