From this guide for laypersons interacting with monks:
The Buddha suggested that the basic source of food for bhikkhus
was that received on the morning alms round (pi.n.dapaata). This daily
dependence on alms food reminds both the bhikkhus and the lay devotees
of their interdependence and prevents the bhikkhu from becoming too
isolated from the lay community. He 'meets' them every day and eats
the food that they share with him. Several important rules are
concerned with this as well as a major section of the Sekhiya Training
rules.
An alms round is not considered begging, for the bhikkhu does not
solicit anything but is ready mindfully to receive any alms that lay
people may wish to give. Although alms food may sometimes be meager,
the bhikkhu is always expected to be grateful for whatever he is
given. It is surprising how particular we can be about what food
we like to eat; and what complications that can cause. This is
reflected in the way rules concerning 'edibles' are arranged, which
may seem very complex especially when the bhikkhu's life is supposed
to be so simple. It should be borne in mind that the rules often deal
with extraordinary circumstances and try to prevent them from becoming
the norm.
However, there are the exceptions called the Great Standards which may allow monks in non-Buddhist countries to bend the minor rules by cooking or farming:
Already during His lifetime, the Buddha made special allowances for
different regions (or desa) outside the 'Middle Country' of North
India — where He lived and taught. These dealt with both the workings
of the Community — for example, a smaller quorum for ordination is
allowed in distant parts where there are fewer monks — and practical
measures, such as special dispensation for footwear and bathing. (See
EV,II,p.173) So there is a precedent for adapting to conditions, but
this does not mean the abolishing of any rules [see End Note 6].
The Lord Buddha also left us a set of principles that can still be
used as a standard to judge new circumstances. These are known as
'The Great Standards.' Properly used they should protect against a
wholesale dilution of the Rule.
This is how the Great Standards are formulated:
"Bhikkhus, whatever I have not objected to, saying, 'This is not
allowable,' if it fits in with what is not allowable, if it goes
against what is allowable, that is not allowable for you.
"Whatever I have not objected to, saying, 'This is not allowable,' if
it fits in with what is allowable, if it goes against what is not
allowable, that is allowable for you.
"And whatever I have not permitted, saying, 'This is allowable,' if it
fits in with what is not allowable, if it goes against what is
allowable, that is not allowable for you.
"And whatever I have not permitted, saying, 'This is allowable,' if it
fits in with what is allowable, if it goes against what is not
allowable, that is allowable for you." (BMC p.27; see also EV, II,
p170)
Treated with care, these Great Standards should enable bhikkhus to
live according to the Vinaya Rule in, for example, isolated
communities in non-Buddhist countries with non-tropical climates. They
form a touchstone for modern conditions and substances.
The Accayika Sutta talks about the most urgent duties of a monk:
"There are these three urgent duties of a farming householder. Which
three?
"There is the case where a farming householder quickly gets his field
well-plowed & well-harrowed. Having quickly gotten his field
well-plowed & well-harrowed, he quickly plants the seed. Having
quickly planted the seed, he quickly lets in the water & then lets it
out.
"These are the three urgent duties of a farming householder. Now, that
farming householder does not have the power or might [to say:] 'May my
crops spring up today, may the grains appear tomorrow, and may they
ripen the next day.' But when the time has come, the farming
householder's crops spring up, the grains appear, and they ripen.
"In the same way, there are these three urgent duties of a monk. Which
three? The undertaking of heightened virtue, the undertaking of
heightened mind, the undertaking of heightened discernment. These are
the three urgent duties of a monk. Now, that monk does not have the
power or might [to say:] 'May my mind be released from fermentations
through lack of clinging/sustenance today or tomorrow or the next
day.' But when the time has come, his mind is released from
fermentations through lack of clinging/sustenance.
"Thus, monks, you should train yourselves: 'Strong will be our desire
for the undertaking of heightened virtue. Strong will be our desire
for the undertaking of heightened mind. Strong will be our desire for
the undertaking of heightened discernment.' That's how you should
train yourselves."