Japanese sentences often assume a certain degree of contextual knowledge. For example, if you say, "I went" in Japanese, you can simply say:
行きました。Ikimashita.
You don't have to include the pronoun "I" as in:
私は行きました。 Watashi wa ikimashita.
While the latter sentence would be perfectly grammatical, it would be unnecessarily emphatic in most cases.
Similarly, suppose that a sales manager and his sales team are discussing the allocation of responsibility for the company's various customers. The sales manager might say something like:
このお客さんは、だれが。。。? Kono o-kyaku-san wa dare ga.....?
The omitted predicate might be something using the verb 担当する (たんとう する), so that the complete sentence thought would be:
このお客さんは、だれが担当していますか。Kono o-kyaku-san wa dare ga tantoo shite imasu ka? (Who is in charge of this customer?)
However, Japanese doesn't require you to spell every word out so emphatically. If people already have an idea of what you are talking about, then it is often OK to omit part of the sentence.