The https://universaldependencies.org/u/dep/iobj.html guidelines give some general principles but can be improved:

• the term "indirect object" can have many different interpretations outside of UD, which should be highlighted

• more examples from different languages

• per agreement of the core group, amend the policy for a sole overt object, which is currently problematic (especially considering cases with an object+ccomp). (Previous discussion in Errors in edeps: double subjects/objects; and iobj vs. obj when only the recipient argument of a ditransitive verb is overt UD_English-EWT#256, iobj/obj (with ccomp and xcomp) consistency in matrix verb annotations UD_English-EWT#55, some of Recommended treatment of internal objects (a.k.a. cognate objects) #832, etc.) The final portion starting with

  In general, if there is just one object, it should be labeled obj, regardless of the morphological case or semantic role. For example, in English, teach can take either the subject matter or the recipient as the only object, and in both cases it would be analyzed as the obj:

  needs to be rewritten with iobj allowed if an obj can be inserted or a complement clause is present.