MIŞ + DIR   Form can be used for both certainty and possibility.

Verb Stem + mIş + [personal suffix] + dIr

Evden  çık    +     mış  +  tır.[note that "d" changes into "t"]

Evden  çıkmıştır.

He/She must [may] have left home.

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Seni  duymuştur.

He/She must [may] have heard you.

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Yeni evlerine yerleşmişlerdir.

They must [may] have settled in their new  house.

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Bu saatte  uyumuştur bile.

He/She must have already gone to bed at this hour.

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The same form can be used in con ditional sentences, as well :

İstanbul´a gittiyse , Sultanahmet Camiini görmüştür.

If he/she went to İstanbul, he/she must have seen Sultanahmet Mosque.

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O filmi izlediyse, çok hoşuna gitmiştir.

If he/she watched that fılm , he/she must have enjoyed it alot.

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Mektubu okuduysa, ne demek istediğimi anlamıştır.

If he/she read my letter , he/she must have understood what I meant.

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Ankara´ya gittiyse ,arkadaşıyla buluşmuştur.

If he/she went to Ankara, he/she must have met his/her friends.

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Interestingly, it is a real condition in the past. The speaker just doesn´t know if the action took place or not. If it was unreal it would be something like

                  İstanbul´a gitseydi, Sultanahmet Camiini görürdü / görmüş olurdu 

´If she had gone...she would have seen...´, true?

Conditional clauses are an endless source of wonders.

Tunci in my opinion there is no certainity in the second part too. They are guesses too.

Gökuyum, do you mean the conditional sentences ? or the previous one ?

Conditional ones. How can we be sure, it is still a guess.

By saying this form can be both "certainity [must]" and "possibility[guess] [may]"  I meant the first group.

when it comes to conditional one, in my opinion this form works same way, as we take one of the example ;

Mektubu okuduysa, ne demek istediğimi anlamıştır.

If he/she read my letter , he/she must have understood what I meant.

Certainity ---> I am so sure [certain] that he/she will understand the message that I want him/her to understand when she reads the letter. I can be sure that she will understand because I know her and her understanding.

Possibility ---> also it is highly possible [I guess] he/she will understand my message in the letter.

IF he/she read the letter.

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we can put "kesin" before the verb to make it more certain.

kesin anlamıştır.

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This is my understanding..However, as always, I respect your valuable opinions Gökuyum.