What is the difference between a squad army and a regular army?

In medieval Russia, the main army was the prince’s squad and the militia, which gathered at the moments necessary for the defense or attack on the enemy. In their free time, the warriors occupied themselves with their lands, received as a patrimony from the prince for military merits. The regular army appears in the 17-18 century, replacing the archers and squads. Its main difference is that its members are constantly engaged in combat training and serve, receiving a salary for this over the course of many years.