English wording should be "the object should be maintained in a fixture", the writer took the freedom of abbreviating this creating "fixtured".
"l'objet devrait être fixé" could imply some damaging methods: nails, glue, etc; here the idea is that there is a need to have the object maintained by something else than hands and the own stability is not sufficient
I guess this is to measure precisely the object, holding in hands increases the temperature which is not desirable, and sometimes is impractical if one hand holds the micrometer and the other one turns the wheel...