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InDesign is a program designed for people used to typesetting terms like p for pica and such things.
It's not inherently clear in the dialog box on how to change this.
Firstly, if you want to use MM all the time then with no documents open, go to Edit>Preferences and choose Units and Increments and Set to MM.
And if you want to - InDesign lets you type ANY measurement into fields with dimensions, like you can type 210mm, or 21cm, or 8.26772inches, or 46.54 ciceros
You can even combine measurements using things like "210mm+1cm" or "210mm+2inches"
You can set any meassurment with the number in a length field, so instert the value in mm. Sometimes it happens that you insert a whole value, let's say 100mm and you end up in 99,99 or 100,01mm. Or you set up an left inset of 20mm and a first line intent -20mm and the programme will tell you that you cannot use values as these. Both comes from internal roundings which are caused because the program uses dtp points to calculate everything.
To change the meassurments to metric go to the preferences panel and change it to one of the metric (cm or mm). If you do it when no document is open it will change it for all documents you create new on this computer, not for existing documents, if you do it when a document is open, it will change it for this document bur not for others.