what happens in the case of uncertainty?
Where there is not sufficient clarity, the trust may be held void as uncertain and give rise to any the three uncertainities i.e."Conceptual uncertainty”, "Evidential uncertainty” and “Ascertainability"
Three certainties
Where there is not sufficient clarity, the trust may be held void as uncertain. The applicable forms of uncertainty have been categorised as:

Conceptual uncertainty is the "most fundamental in the validity of a trust or power", and is where the language used in the trust is unclear. Examples include where familiar but overly vague terms are used, such as "good customers" or "useful employees"; if the concept cannot be certain, the trust fails. Evidential uncertainty, on the other hand, is where there is a question of fact it is impossible to answer, such as when a claimant cannot prove he is a beneficiary. This does not necessarily invalidate the trust, as Jenkins J (as he was then) said in "Re Coxen":
The next type of uncertainty, ascertainability, is where it is impossible to find the beneficiaries, either because they have died, moved or changed names. This is not necessarily fatal; the test for deciding if it is or not was laid out by Wynn-Parry J as: "mere difficulty of ascertain
inland revenue commissioners v broadway cottages trust 1955 ch 20 It states that if there are any potential beneficiaries who the trustees are not certain of, or the trustees cannot compile a complete list, the trust is void for uncertainty. what is this three certainties all about? It refers to a rule within English trusts law on the creation of express trusts that, to be valid, the trust instrument must show certainty of intention, subject matter and object. what is the above-mentioned instrument all about? It is an instrument in writing executed by a settlor used to constitute a trust. who is the settlor in this case? He is a trustor, or occasionally, a grantor or donor. who created these certainties? UNANSWERABLE list those three certainties. Certainty of intention", "Certainty of subject matter" and "Certainty of object". what does the first certainty state? To create an express trust without being aware that one is doing so, so long as the court can determine from the person's intention that a beneficial entitlement should be conferred which the law will enforce. what does the second certainty state? It is a requirement that the subject matter be certain — that the property intended to be in the trust be separated from other property, showing clarity in what is intended to be trust property. If there is no clear separation, the trust will fail. what does the third certainty state? There is a requirement that the beneficiaries of a trust, known as the objects, be certain. Within express trusts this is a particularly complex area, because the test used to determine certainty varies between fixed trusts, mere powers and discretionary trusts.
wiki:9190488
Three certainties [SEP] Uncertainty
Where there is not sufficient clarity, the trust may be held void as uncertain. The applicable forms of uncertainty have been categorised as: Conceptual uncertainty is the "most fundamental in the validity of a trust or power", and is where the language used in the trust is unclear. Examples include where familiar but overly vague terms are used, such as "good customers" or "useful employees"; if the concept cannot be certain, the trust fails. Evidential uncertainty, on the other hand, is where there is a question of fact it is impossible to answer, such as when a claimant cannot prove he is a beneficiary.