Following on Ven. Sobhana’s post in the Kelly Class 5 thread…
For context, in Lesson 2, G&K use AN 3.19 as a teaching aid:
Idha, bhikkhave, bhikkhu pubbaṇhasamayaṃ sakkaccaṃ samādhinimittaṃ adhiṭṭhāti, majjhaṇhikasamayaṃ…
This gave rise to a side discussion during Class 6 on the precise meaning of samādhinimitta. We landed, more or less, on objects of concentration.
The more I thought about samādhinimitta, I started delving into the term. How would one practice samādhinimitta all day long? Concentration on what objects? If we’re not talking about the breathing process, then what are we talking about?
I contemplated the practicality of placing my attention on coloured disks (for the umpteenth time). I’ve sat, forced my attention on a contrived coloured circle in my mind, and nothing “happens” except that I’m aware I’m doing it.
I can’t imagine meditating that way. Back to the breath.
I found my way to this essay by Bhikkhu Sona: " THE MYSTERY OF THE BREATH NIMITTA (OR THE CASE OF THE MISSING SIMILE)
In it he writes:
[Mindfulness of in-and-out breathing] is described in a number of suttas belonging to the Pali Canon (e.g., M 118, M 10, D 22). However, the suttas are quite concise, and at times sparse, in their treatment of meditation methods. Hence, one finds post-canonical exegetical works having as their main purpose the comment, explanation, complement, or clarification of texts that may be deemed abstruse or lacking information within the Canon.
He goes on to clarify some things around the use nimitta as it relates to anapanasati. This was very helpful.
Then I found my way to a 2017 (!) essay on Discuss & Discover:
It’s a bit of a slog to get through; it’s a thorough study on the use of the term nimitta in the EBTs. I’ll be reviewing it again.
Which brought me, full-circle, to Bhante Sujato’s translation of AN 3.19:
In the same way, a mendicant who has three qualities is able to acquire more skillful qualities or to increase the skillful qualities they’ve already acquired. What three? It’s when a mendicant carefully applies themselves to a meditation subject as a foundation of immersion in the morning, at midday, and in the afternoon.
OK, now I’ve moved from object to subject. Still not so clear but I’m getting the drift we’re not looking for specific instructions in the EBTs. Guess it’s time to just let it go for now. 
Isn’t studying pāli great?