I had been holding out on this question in hopes that someone else would answer it. This has not happened. I apologize. I will begin by recommending two texts that will answer the second part of your question.

1. Charles Haring - The Spanish Empire in America

2. Mario Gongora - Colonial Spanish America

These texts are rather dated, but until the 1980s they were the go-to texts for understanding the essential framework of the Spanish Empire in the Americas. What happened? Various things. Many scholars began assigning their graduate students more complex authors and lectured on the basic functions of the empire. Their students, the professors of the 1990s and early 2000s, then passed on their basic knowledge to their students. They understood rather complicated elements of political, religious, and social issues, but did not explain fully the quotidian operations of that empire. In the past few years, scholars have begun introducing these older scholars into their research, hopefully we can have new scholarship that builds upon these older researchers and historians. A new generation of scholarship is necessary to not only pass this information on, but also to build upon the giants of the early scholarship. We are all descendents of Moses and Bolton. It would be nice to reference them once in a while.

To answer the primary question, the duties of the maestre de campo varied from capitan-general to capitan-general. There is some tendency to associate this position with a chief-of-staff. At some times, this is exactly the case. While the Prussians made the staff system efficient in the modern era, there is evidence that some imperial systems in the Americas had rather complex staff hierarchies as well. This was not the norm, however. In Europe, the title carried with it the weight of a field commander of troops. In the Americas, where an organized army was rarely established until the revolutionary period, the title was more combined with alcalde ordinario. As a frontier political commander, the frontier military commander role often overlapped. It was easier to make frontier personnel embody the same positions to save money on salaries and maintain order. In the Caribbean and the frontier regions of the Spanish Main in the first half of the sixteenth century, this model prevailed. See David Wheat's latest book to see how this operated in more detail. In later years, say Peru in the late 1700s, the title was more closely tied to that of a commander or regularly troops on limited detail. There is some work being done by scholars of Revolutionary Peru, Ecuador, and Upper Peru that seem to follow this model.

To answer the first question more succinctly: multiple and various depending on the time and place.