There is a distinction between transcriptions and arrangements. Why is this important to know? Many of the pieces performed by tuba-euphonium ensembles are called arrangements, but upon closer inspection, one might label some of them transcriptions. In my opinion, the two are different and are summarized below!
These are new configurations of a pre-existing work. It is placed in the context of a new ensemble setting (i. e. from choir to wind ensemble). There is usually something unique about the arrangement. The arranger can make his or her own introduction, transition materials, or a new ending. Also, the pre-existing work can be worked into a new context. For example, one of my final projects in my undergraduate band arranging course was to arrange a Christmas tune for full band. I arranged In dolce jubilo. The arrangement began with a full band, chorale-style rendition of the tune. I then inserted my own modulation to get from the home key of B-flat major to E-flat major. After this modulation, I reworked the tune into a bright, lively march! Yes, complete with oom-pa accompaniment. Within the march, I added a modulation to A-flat major and wrote a colorful woodwind line above the brass section. The closing section was an augmentation of the march melody, closely resembling the opening of the piece, and closed with a stinger. My undergraduate professor said that arrangments have to bear the mark of the arranger. The person makes it his or her own by making the piece different. What that difference is varies from piece to piece.
These works also stem from pre-existing materials. This happens a lot in band literature. Many pieces are from the orchestral repertoire. In the case of a tuba-euphonium ensemble, many four-part choir works transcribe well for the tubas and euphoniums. The soprano, alto, tenor, and bass parts translate to euphonium 1, euphonium 2, tuba 1, and tuba 2 very well. There are few, if any, changes in structure to the original work. Transcriptions are essentially the previous work in the guise of another ensemble. Take the Forbes version of Achieved is the Glorious Work. This piece is a chorus from Handel's The Creation. There is a four-part trombone version of this piece, which Forbes then reworked for tubas and euphoniums. This reworking likely deserves the term transcription and not arrangement.
I know it may seem like we're losing words and getting hung up on little differences, but the fact remains that we have these two different terms with two different meanings. The transcription police will not be called if you call it an arrangement! Many publishers market items as arrangements, so bless it be and call it a day!