In the text, Esther 9:15 reads (in part)
the Jews in Shushan mustered again on the fourteenth day of Adar and slew three hundred men in Shushan...
This puts them them, those in Shushan, as separate from other Jews in "provinces" who, according to 16-17
The rest of the Jews, those in the king’s provinces, likewise mustered and fought for their lives...That was on the thirteenth day of the month of Adar; and they rested on the fourteenth day and made it a day of feasting and merrymaking
So it seems that the continued fighting was only in Shushan (9:18)
But the Jews in Shushan mustered on both the thirteenth and fourteenth days, and so rested on the fifteenth, and made it a day of feasting and merrymaking.
But when the proclamation regarding the establishing of a Purim celebration (9:20) was sent to all the provinces (medinot), near and far, the text records in 9:21
לְקַיֵּם֮ עֲלֵיהֶם֒ לִהְי֣וֹת עֹשִׂ֗ים אֵ֠ת י֣וֹם אַרְבָּעָ֤ה עָשָׂר֙ לְחֹ֣דֶשׁ אֲדָ֔ר וְאֵ֛ת יוֹם־חֲמִשָּׁ֥ה עָשָׂ֖ר בּ֑וֹ בְּכָל־שָׁנָ֖ה וְשָׁנָֽה
that the people in the provinces would celebrate on both the 14th and 15th. Since Shushan, the capital city, wasn't considered one of the provinces earlier on, there must have been places outside of Shushan that would mark the 15th as the holiday.
What places would have been the object of that part of Mordechai's edict? Do we have names of cities, perhaps?