English militias have been in existence, in various forms, for centuries. By the middle of the eighteenth century it became clear that a radical reform of the system was required if the militia were to be an effective defence force. There followed a number of Militia Acts starting in 1757. The militia continued to be raised through a ballot. Men were to be chosen by ballot among the able-bodied men of the parish between the ages of 181 and 50, and would serve for three years (later to be extended to five). If they did not wish to serve, they could either provide a substitute or pay a £10 fine. From the Militia Act 1802, there were to be 5 levels of candidate for the ballot, meaning that if the first category failed to raise the required number of men in a particular year, those in the next level would be balloted and so on.
Men were divided into five classes:
Under 30, no children
Over 30, no children
All men, no living children under 14
All men, one child under 14
All other men
The way in which a man could volunteer was to offer himself as a substitute for a man chosen in the ballot who was prepared to pay him to take his place. Thus the volunteer not only received the bounty of 2 guineas on joining, but also a sum of money for acting as a substitute. When not embodied, service in the militia was not onerous. They had to attend 21 days initial military training and thereafter attend for two week's annual training.
However the threat to Britain caused by the Revolutionary Wars in France necessitated enlarging both the militia and the Regular Army, and so the militias were opened up directly to volunteers, and the number of men required through the ballot reduced accordingly. Effectively the militia was then divided into two parts, the so-called Regular militia, and the local militias. The Regular militia were to act as a reserve for the Regular Army, while the local militias continued to be intended solely for defence within the United Kingdom (by this stage Scotland and Ireland had their own separate militia forces).
From the Militia Act of 1796 the strength of the combined militia force was authorised at 120,388 men.
When embodied, a militia man would be paid at the same rate as a regular soldier and, in theory at least, was subject to the same discipline. But since all the officers and men in a militia company were effectively civilians in uniform, discipline was much more relaxed than would have been the case in the Regular Army. When embodied the militia were billeted in inns and occasionally on the local population, and more often housed in tented camps, when away from their own county.
So it is not possible to say, without more information, whether William Neal was chosen by ballot or was a volunteer.
Footnote
1. The lower age of 17 was introduced in 1803 because of the difficulty in recruiting the full numbers required; prior to that the minimum age was 18. A 17 year old had to be over 5' 2" and have, or be likely to have by his 18th birthday, a fully developed adult male physique.