The gemara is trying to find the case where a lost gett is returned to the woman without being concerned that it is not her gett being given to her. R' Ashi makes the case to be where it is returned to her by her giving a "siman muvhak" such as identifying a hole next to a particular letter in the gett. The siman must be an excellent siman to work on something d'oraysa such as gett, because standard simanim are questionable whether they are effective d'oraysa or only d'rabonon.
The Pri Megadim in his rules of simanim and tevi'as ayin (printed at the end of his commentary on the first chelek of y.d.) says that their are 3 types of simanim:
1. Siman Muhak - There are very specific type of simanim such as a hole next to a particular letter. All agree that simanim of this type are d'oraysa and therefore work even by a gett. Furthermore, this type of siman works even to take away from a chazaka, just as it removes a woman from a status of eishes ish. Although this would seem obvious from our gemara, it isn't. The pri megadim raises the possibility that on a d'oraysa level we aren't concerned that this gett belongs to someone else (even if there are husbands with the same names and the caravans are common), the entire concern of it belonging to someone else is only d'rabonon. Therefore, it could be that although siman muvhak does not have the power to remove from a chazaka, it works since the entire concern of the gett belonging to someone else is only d'rabonon.
2. Standard siman - This is a machlokes whether it is d'oraysa or d'rabonon. Rambam paskens that it is d'oraysa based on the gemara later 28a. If we assume a standard siman is d'oraysa, why can't we rely on it to testify for a woman that her husband died? Pri Megadim explains that even if it is d'oraysa it isn't strong enough to take out of a chazaka, therefore it only works d'oraysa by a lost object, but not to allow a woman to remarry.
3. Poor simanim - This group is a type of siman that is barely recognized as a siman even on a d'rabonon level. It is only in the context of a low level issur d'rabonon such as meat that has been left unattended, do we rely on this type of siman to confirm that it is the same meat that was left originally.