The matter is disputed whether classical Latin employed subjunctive on its own for polite requests. To make a polite request in classical Latin to an equal or superior, one could use a phrase such as obsecro, quaeso, or amabo. Here's a link: https://www.latinitium.com/blog/politeness-in-latin Maybe DL Latin is exercising an ellipsis here of some sort. Our sources are often from elites, so in texts such as Petronius' Satyricon, the imperative is most common, because slaves, often referred to as pueri, are being ordered around. Although Rome was a slave culture, there's a lot we don't know about who took food orders at a taverna or popina. There's not a lot of deferential language or polite language in a such satirical text from an elite (Patronius). However, Jerome often inserts deferential language into his translation when someone is speaking to a superior, as is the case Gen 12:13 dic ergo obsecro te quod soror mea sis ut bene sit mihi. Notice the subjunctive use there is in clauses following quod and ut. Gen 19:2 obsecro domini declinate in domum pueri vestri, "I pray, Lords, turn aside to the house of your slaves." The use of the imperative is thereby toned down with obsecro. Jerome often inserts deferential language in specific circumstances. Outside the context of an elite triclinium, it's more difficult to know how someone would order food at at a cheap street taberna or popina. They likely would have used the imperative or indicative, although if a low ranking person was speaking to a superior, he might add obsecro, quaeso, or amabo, as well as velim, + ut, in a specific context provided the person taking the order was not a slave. If we could spend a day observing people ordering in a taverna in Rome or Pompey, we would find out what sort of phrases they actually used. I simply want to raise a caveat: DL Latin is filling in a blank in a way that makes this Latin course amenable to people who want to approximate speaking Latin today rather than based on firm evidence that Romans actually used velim on its own in this way.