Regarding Samādhinimitta, these are my notes:
Sujato’s translation: a meditation subject as a foundation of immersion
Bodhi: an object of concentration
Culavedallasutta MN44 :
“Yā kho, āvuso visākha, cittassa ekaggatā ayaṃ samādhi;
“Unification of the mind is immersion.
cattāro satipaṭṭhānā samādhinimittā;
The four kinds of mindfulness meditation are the foundations of immersion.”
Based on MN 44, I think of samādhinimitta as: “the foundations for unification of mind”
In relation to the phrase in MN44, here is Analayo’s comment:
On considering these instances it is indubitably clear that sati has a crucial role to fulfill in the realm of samatha. This might be why the Cūḷavedalla Sutta speaks of satipaṭṭhāna as the “cause” of concentration (samādhinimitta). On the other hand, however, to consider satipaṭṭhāna purely as a concentration exercise goes too far and misses the important difference between what can become a basis for the development of concentration and what belongs to the realm of calmness meditation proper. In fact, the characteristic functions of sati and concentration (samādhi) are quite distinct. While concentration corresponds to an enhancement of the selective function of the mind, by way of restricting the breadth of attention, sati on its own represents an enhancement of the recollective function, by way of expanding the breadth of attention. These two modes of mental functioning correspond to two different cortical control mechanisms in the brain. This difference, however, does not imply that the two are incompatible, since during absorption attainment both are present. But during absorption sati becomes mainly presence of the mind, when it to some extent loses its natural breadth owing to the strong focusing power of concentration.”
― Anālayo, Satipaṭṭhāna: The Direct Path to Realization
Here is Piya Tan’s comment:
“The Basis of samādhi. Here nimitta is best taken in the sense of “means,” since it refers to conditions for mental focus. — Piya Tan, Nimitta Sutta
Commentary for this sutta is explained by Piya Tan. It can refer the nutriment of sāmadhi, especially the nutriments for the samādhi sambojjhanga! https://www.themindingcentre.org/dharmafarer/wp-content/uploads/2010/02/37.2a-Papanika-S-1-a3.19-piya.pdf