[BIGLOU56]

What differentiates styles of beer, say in a porter (9C, 13C, 20A)?

[Richard]

From the 2015 BJCP guidelines, the overall impressions for these three are:
9C) A Baltic Porter often has the malt
flavors reminiscent of an English porter and the restrained
roast of a schwarzbier, but with a higher OG and alcohol
content than either. Very complex, with multi-layered malt and
dark fruit flavors.
13C)A moderate-strength brown beer with a
restrained roasty character and bitterness. May have a range of
roasted flavors, generally without burnt qualities, and often has
a chocolate-caramel-malty profile.
20A) Overall Impression: A substantial, malty dark beer with a
complex and flavorful dark malt character.

The style guidelines are here: https://www.bjcp.org/stylecenter.php

[Kevin]

If you really want to dive deep into the world of Porter... at least historically... you must read Ron Pattinson's Blog; Shut Up About Barclay Perkins. Do a search on the site for Porter and you will find almost 10 pages of articles. Then search London Porter and find 8 or 9 pages more. http://barclayperkins.blogspot.com/

[James K]

For me the big thing in the specific styles you mentioned is abv and ibus.
Baltic Porter has the highest abv and American porter allows for the highest bitterness. Also with Baltic Porter you’re looking for the alcohol complexity to balance the beer more so than a malty balance. English styles are usually more bisquit and bready characters while American styles are more malty.
The last thing is the final gravity. The Baltic Porter is much more of a full beer and less dry than the other two, it says a lager strain which likely means a beer that has more care and time put into it, thus the complexity can come out over a long period of time.

[KellerBrauer]

Another great reading is: Porters and Stouts, by Terry Foster.