Is it just a fancy name for the Poylishe minhogim, or what? Is any Nusach Sfard siddur automatically minhog Polin or is it more specific?
Minhag Polin refers to liturgical variations within the Ashkenazi community which differ from the practice in Germany. Current mainstream Ashkenazi practice is close to Minhag Polin (because the community in Poland and their descendants demographically overwhelm German Jews). So, though Askhenaz literally refers to Germany, the specific customs of Germany (Minhag Ashkenaz) are the exception, not the rule, in the current Ashkenazi world.
Examples of Polin / Ashkenaz variants (Minhag Polin listed first):
- Birkat Ha-Torah immediately following Elohai Neshama vs. Birkot Ha-Shahar immediately following Elohai Neshama, followed then by Birkat Ha-Torah.
- Psukim of Birkat Cohanim as the material for learning after Birkat Ha-Torah vs. Parashat Ha-Tamid as the material for learning.
- Shalom Rav vs. Sim Shalom in the Amidah of Shabbat Minhah.
Nusah Sfard is a separate issue and refers to variations applied by the Hasidim to Askhenazi liturgy.
Minhag Poylin pretty much refers to the Minhogim of central polish chassidus. Also Nusach Sefard is not automatically Poylish. Nusach Sefard is based of Nusach haAri"zal with certain motifications by the Baal Shem Tov Ztz"l and his Talmidim.