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http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FEAT/UserGuide
From FIRST user guide:
randomise has the following thresholding/output options:
Voxel-based thresholding, both uncorrected and corrected for multiple comparisons by using the null distribution of the max (across the image) voxelwise test statistic. Uncorrected outputs are: <output>_vox_p_tstat / <output>_vox_p_fstat. Corrected outputs are: <output>_vox_corrp_tstat / <output>_vox_corrp_fstat. To use this option, use -x.
TFCE (Threshold-Free Cluster Enhancement) is a new method for finding "clusters" in your data without having to define clusters in a binary way. Cluster-like structures are enhanced but the image remains fundamentally voxelwise; you can use the -tfceoption in fslmaths to test this on an existing stats image. See the TFCE research page for more information. The "E", "H" and neighbourhood-connectivity parameters have been optimised and should be left unchanged. These optimisations are different for different "dimensionality" of your data; for normal, 3D data (such as in an FSL-VBM analysis), you should just just the -T option, while for TBSS analyses (that is in effect on the mostly "2D" white matter skeleton), you should use the --T2 option.
Cluster-based thresholding corrected for multiple comparisons by using the null distribution of the max (across the image) cluster size (so passé!): <output>_clustere_corrp_tstat / <output>_clustere_corrp_fstat.
To use this option, use -c <thresh> for t contrasts and -F <thresh> for F contrasts, where the threshold is used to form supra-threshold clusters of voxels.Cluster-based thresholding corrected for multiple comparisons by using the null distribution of the max (across the image) cluster mass: <output>_clusterm_corrp_tstat / <output>_clusterm_corrp_fstat.
To use this option, use -C <thresh> for t contrasts and -S <thresh> for F contrasts.