Like other masculine Spanish words, masculine Spanish pronouns can be used when the gender of the subject is unknown or when the subject is plural and of mixed gender.
Treated as if it were third-person for purposes of conjugation and reflexivity
If le or les precedes lo, la, los, or las in a clause, it is replaced with se (e.g., Se lo dije instead of Le lo dije)
Depending on the implicit gender of the object being referred to
lo - Wikipédia (spanyol)