Garum, the fish-based saucy condiment ubiquitous in ancient Roman cookery, shows up in many places, such as the epigrams of Martial, the scientific treatises of Pliny, and the medicinal writings of Galen. Astute students of ancient culinary history will have noticed, however, that the cookery book attributed to Apicius tends to use the term liquamen when referring to fish sauce, rather than garum. Other terms denoting fermented fish products are scattered throughout the Greco-Roman record, such as garum sociorum, muria and allec. But what do these terms actually indicate?
Few scholars have the in-depth knowledge of the literature of the classical world and the practical experience in cookery and experimental archaeology to analyze these ancient fermented fish products. 
Few scholars, indeed, except for the inimitable Sally Grainger, who spent the early part of her professional career as a food historian cooking ancient Roman food (indeed, she prepared the tasty fish sauces that we sampled at the pre-talk reception), but she has shifted in recent years to focus more on the academic exploration of Roman cuisine. The focus of her post-graduate work at the University of Reading has been to analyze and experimentally recreate the fish sauces used in Roman cuisine, and she is now one of the world’s authorities. Grainger’s erudite presentation was geared to an audience already sophisticated in the basic contours of Roman cookery: she meticulously reviewed ancient sources, both well-known and obscure, for the clues they offered to the different terms used, seemingly haphazardly, to identify fermented fish sauces.
Her big points were that these classical world’s universe of fermented fish sauces is much greater than the imprecise, catch-all term garum would suggest. Fish sauces fell into two (or three) main categories, depending on their prime constituent–some were made from fermenting only the viscera of fish, others from blood, while still others from whole fish. These viscera- and blood-based sauces were luxury products, exclusively used at table, t while those from whole fish were mass-market. While some of these mass-market products were used as table sauces, many found work only in the kitchen, as an ingredient to make more complex sauces, or as a cooking medium. An analogy might be to the different culinary uses of artisanal ketchup as a table condiment versus puréed tomatoes or cheap ketchup as an ingredient in the kitchen.
Much of the difficulty in understanding these sauces arises from the fact that Pliny, Martial, and other writers used the terms garum and liquamen inconsistently (likely owing to confusion and ignorance by non-cooks in the ancient world over how these products were made). These inconsistencies bedevil the modern scholar hoping to unravel the different terms’ meaning. But Grainger brought an interesting, class-based argument in support of her thesis the literary sources made a muddle of these terms. Although the ancient writers such Martial and Pliny recognized that there were different grades (and thus different prices) of fish sauces, Grainger believes that their class and elite professions meant that they had relatively little exposure to the universe of fish sauce products except those found on well-to-do tables. 
Grainger notes that the cooks and merchants in fish sauces (generally lower-class sorts) had a much more expansive vocabulary for these fish products because they worked with them on a daily basis. The elites who found the best sauces on their tables had little reason to master the subtleties of the different categories of sauces: in this case, the well-educated Pliny suffered from ignorance.
Grainger’s argument is complex and requires careful study to appreciate the literary evidence she assembles to support her view that, contrary to the view of some scholars, garum and liquamen are not interchangeable, nor are they different terms for the same product, with liquamen coming to the fore in the late Empire. Rather, she sees the plethora of terms as the vernacular of the folks who made, processed, distributed, sold, or worked with these ingredients in the kitchen; just as elites nowadays may appreciate fine quality chocolates on the table and call every luscious tidbit a “truffle,” they may know little about the complex terminology that cacao growers, distributors, and chefs use to communicate precisely about their products and ingredients.
To delve deeper, her paper Garum, liquamen, and muria: a new approach to the problem of definition is available for free by registering on academia.edu: https://reading.academia.edu/SallyGrainger.