What does it mean to ask 'what is the subject of this poem'?

Question

I read a poem called "Vigil Strange I ept on the field one night" by Walt Whitman (reproduced below). One question I was asked about this poem is 'what is the subject of this poem?' For my understanding, it seems to ask 'what is the theme of this poem', so my answer is 'friendship and camaraderie in time of war', but it is wrong. The correct answer is 'reminiscence'.

I am wondering what the 'what is the subject of this poem' means. Doesn't it mean 'themes' of the poem?

Vigil strange I kept on the field one night;
When you my son and my comrade dropt at my side that day,
One look I but gave which your dear eyes return’d with a look I shall never forget,
One touch of your hand to mine O boy, reach’d up as you lay on the ground,
Then onward I sped in the battle, the even-contested battle,
Till late in the night reliev’d to the place at last again I made my way,
Found you in death so cold dear comrade, found your body son of responding kisses, (never again on earth responding,)
Bared your face in the starlight, curious the scene, cool blew the moderate night-wind,
Long there and then in vigil I stood, dimly around me the battle-field spreading,
Vigil wondrous and vigil sweet there in the fragrant silent night,
But not a tear fell, not even a long-drawn sigh, long, long I gazed,
Then on the earth partially reclining sat by your side leaning my chin in my hands,
Passing sweet hours, immortal and mystic hours with you dearest comrade—not a tear, not a word,
Vigil of silence, love and death, vigil for you my son and my soldier,
As onward silently stars aloft, eastward new ones upward stole,
Vigil final for you brave boy, (I could not save you, swift was your death,
I faithfully loved you and cared for you living, I think we shall surely meet again,)
Till at latest lingering of the night, indeed just as the dawn appear’d,
My comrade I wrapt in his blanket, envelop’d well his form,
Folded the blanket well, tucking it carefully over head and carefully under feet,
And there and then and bathed by the rising sun, my son in his grave, in his rude-dug grave I deposited,
Ending my vigil strange with that, vigil of night and battle-field dim,
Vigil for boy of responding kisses, (never again on earth responding,)
Vigil for comrade swiftly slain, vigil I never forget, how as day brighten’d,
I rose from the chill ground and folded my soldier well in his blanket,
And buried him where he fell.

Answer 1

Actually it is difficult at times to differentiate between subject matter and theme of a work of art , one dissolves into the other. The only yard-stick is how one presents his case. Rand alThor shows where they mix and mingle.

However, in general, theme is the topic and a subject embodies that topic. For convenience's sake we would limit our discussion to a literary work only. Subject is what the work of art is about. So the subject matter is rather straight forward. For the theme you have to delve deep because subject matter hold the theme and is embodied in it. It may so happen that they may be same as shown by the learned critic in his comment for your example, but not always. Theme is what the author or poet wants to say or mean. In an allegory, subject matter is a means to an end (the theme).

Let us explain. The Selfish Giant by Oscar Wilde is an innocent story of a giant and children (subject matter) but at heart it is about autocracy and communism (theme)— compassion and love where God resides. Or Frost's Stopping by Wood on A Snowy Evening. It is about poets personal experience on a snowy evening, hence its subject matter. But at heart it is about man torn between the rival claims of material world and yearning for aesthetic pursuit, hence its theme.