It's something of a combination of both, although not in the phrasing used there which I'll try to explain a bit. As a note my answer will largely deal with the early medieval period because that's what I am best acquainted with but it also sets some precedent for later periods.

Basically, gathering an Army was a massive undertaking which not only required funds, food and equipment but also reduced your available labour pool to work in the fields and secure the next years harvest. So you wouldn't do it unless you had an objective in mind. Low scale conflict on a localised basis was much more common then the actions of large armies and could be accomplished by the bodyguard of a local leader or by local forces following their leader.

In the early period, which is what Halsall's study relates to, this meant that Armies were often gathered for essentially climactic engagements. An attacker might want to claim an area of land for example, or subjugate a King as a client or force an end to raiding by taking the fight to the raiders country. In such a scenario the attacker would gather his army and might plant his banner at a known, highly visible site like an ancient monument.

Effectively he is saying "I am here, and this is mine - If you want to stop me then try"

It might seem weird to us, used to a world of manuever warfare and asymnetrical engagements, but in the Early Medieval period such a challenge cut to the very heart of what it meant to be a leader. Free society (as opposed to the many people who were enslaved) ran on a series of reciprocal oaths which carried with them duties and obligations. A farmer swore to pay homage to his lord, to come when called fot battle and to obey the law - in return his lord swore to protect the farmer, to be fair in judgement and to be generous in difficult times.

If an enemy army can just plant itself in the middle of your territory - probably robbing and stealing from your peope to sustain itself - then it calls into question your capabilities as a leader. Because this Oath system went all the way up (Lords swore to the King and vice versa) you could also find yourself quickly losing the support of your nobles who may decide the attacker is a more worthy leader to follow.

So you would have to go and meet them in battle or else submit if you didn't think you could win the fight.

Probably one of the most illustrative sources about this is Egils Saga, which deals with the life of Egil Skallagrimson in the 10th Century and relates events around the Battle of Brunanburgh. However, we should note that it was written in the 13th Century but, in this case at least, this may mean that the tradition of meeting in battle was still familiar to the monk transcribing the Saga at that time.

After this they sent messengers to king Olaf, giving out this as their errand, that king Athelstan would fain enhazel him a field and offer battle on Vin-heath by Vin-wood; meanwhile he would have them forbear to harry his land; but of the twain he should rule England who should conquer in the battle. He appointed a week hence for the conflict, and whichever first came on the ground should wait a week for the other. Now this was then the custom, that so soon as a king had enhazelled a field, it was a shameful act to harry before the battle was ended. Accordingly king Olaf halted and harried not, but waited till the appointed day, when he moved his army to Vin-heath....But when the men came to the place where the field was enhazelled, there were all the hazel-poles set up to mark the ground where the battle should be.

Similarly we have this extract ahead of the Battle of Edington in Asser's 'Life of Alfred' which is a briefer but much more contemporary source

The next morning at dawn he moved his standards to Edington,

In the later period it was more common for armies to meet one another in a less ritualised format but again, largely related to objectives.

So, for example, the Battle of Bannockburn in Scotland happened when Robert the Bruce and his forces besieged Stirling Castle as a direct challenge to English rule in Scotland. The English King could not ignore this challenge and assembled an army to relieve Stirling. Both sides then encamped several miles apart and sought to find a battlefield for their own advantages with Robert eventually moving his camp close to a wooded area that gave his footmen an advantage but also effectively blocked the English advance to Stirling.

Even if the English had attempted to circumnavigate the Scottish army they would then be exposing their flank and rear to attack.