The TT_N27 dataset is in Talairach space. The "TT" stands for Talairach-Tourneaux. The TT_N27 dataset was created by manually talairaching (marking and transforming the stereotaxic coordinates) of the MNI version of the N27 dataset. AFNI is distributed with versions of the N27, MNI_152 and MNI_452 templates and the Eickhoff-Zilles atlases in Talairach, MNI and MNI_ANAT space, where MNI_ANAT space is the version of MNI space that has been shifted for (0,0,0) to be at or near the anterior commissure. In AFNI, you can use TLRC, MNI, MNI_ANAT or any other space. Whether you do your analysis in MNI, TLRC or MNI_ANAT space is not terribly important. The size difference among the templates are fairly small. No matter which space you use, the whereami function provides results in a list of requested spaces.

The @auto_tlrc program followed by 3dMean would be one way to create a template from many subjects.