Although TSS appears to be a straightforward measure of particulate weight obtained by separating particles from a water sample using a filter, it suffers as a defined quantity from the fact that particles occur in nature in essentially a continuum of sizes. At the lower end, TSS relies on a cut-off established by properties of the filter being used. At the upper end, the cut-off should be the exclusion of all particulates too large to be "suspended" in water.  However, this is not a fixed particle size but is dependent upon the energetics of the situation at the time of sampling: moving water suspends larger particles than does still water. Usually it is the case that the additional suspended material caused by the movement of the water is of interest.
These problems in no way invalidate the use of TSS; consistency in method and technique can overcome short-comings in most cases.  But comparisons between studies may require a careful review of the methodologies used to establish that the studies are in fact measuring the same thing.
TSS in mg/L can be calculated as:
: (dry weight of residue and filter − dry weight of filter alone, in grams)/ mL of sample * 1,000,000