One is Ajal Musamma (specified or certain time of death) and the other is Ajal Mu’allaq (suspended time or term). It refers to an indeterminate and changeable time of one's death.
God says in Verse 2 of Surah Al-Anaam: “It is He who has created you from clay to live for a life-time and the span of your life (Ajal Musamma) is only known to Him. You are still in doubt!”
Ajal Musamma is the death that comes when one's ability to survive ends, and when it comes, everything ends by the order of God.
Some verses of the Quran, like Verse 34 of Surah Al-A’raf, “All people can only live for an appointed time, when their term ends, they will not remain (alive) even for a single hour, nor will they die before the appointed time,” point to this kind of death.
But Ajal Mu’allaq is the time of one’s death in accordance with circumstances. It is subject to change, that is, it can be hastened or postponed depending on external factors. For example suicide is a kind of Ajal Mua’llaq because if one does not commit suicide, he may live for many more years.
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Things may have the ability to survive for a long time because of their natural and innate capacity, but before that comes to an end, some event may prevent them from reaching the maximum of their life span. For example, a kerosene lamp may have the ability to burn and give light for 20 hours, but a gust of wind or rain may put it out and shorten its life.
Here, if there is no external factor that shortens its life and the lamp uses up the last drop of kerosene before it goes out, it reaches its certain death, but if some event causes it to go out before that time, its death would be Ajal Mu’allaq ( suspended or uncertain death).