as-Saff is interpreted in two ways:
1. as-Saffen is used in the sense of collectively / all together, as in the following verse:
"Saffen {that is, all together/collectively} they will be presented before your Lord." (al-Kahf 18:48)
This is also the case in the verse in Sūrat Ṭā Hā.
2. Saff is used in the sense of 'row' itself, as in the following verses:
"Indeed, Allah loves those who fight in His cause in a row as though they are a [single] structure joined firmly {that is, joined together like the bricks of a building; here, the rows of believers during battle are meant}." (as-Saff 61:4)
"When your Lord comes and the angels are lined up in rows {that is, the angels in ranks; on the Day of Resurrection, when each group of the inhabitants of the heavens is arrayed in a separate saff (row)...}" (al-Fajr 89:22)