Byakue Costumes (for pilgrimage) (白衣 (巡礼用品))
Byakue costumes are white jackets worn when people make pilgrimage of sacred ground. Similarly as a happi coat (a workman's livery coat) they usually have sleeves; some do not have sleeves as vests.
Summary
Originally they were called oizuru (sleeve-less costume to wear for pilgrimage so that the statue of the Goddess of Kannon on the back will not directly touch the body). In the past people walked with Kannnon-zo (statute of the Goddess of Kannon) on their back during pilgrimage. They put a piece of white cloth between their back and the Kannon-zo so that the statute would not come into direct contact with their clothes. The color white expresses a heart without impurities, as well as the elimination of various worries, doubts, and contamination.
Later in the modern days people started to regard Byakue costumes as replacement of shrouds, burial outfit. This is because severe journey of pilgrimage is next to death. They wear burial outfit so that they can be buried wherever they may die. Or it expresses that they are in the next world (sacred world) in burial outfit, and they make pilgrimage of sacred ground as a man who once died and return with a new life. This is the basic idea of wearing shrouds. Pilgrims wear tekko (covering for the back of the hand and wrist) and kyahan (gaiters) for the same reasons as burial outfit.
Han-i is Byakue costume; it is not worn during pilgrimage but for receiving read seals at fudasho (temple where amulets are collected). When yourself or the one who is close to you die, it is said the person can go to the Buddhist paradise when he or she wears han-i with red seals as burial outfit.
Letters printed on the back of clothes
There are plain Byakue costumes, but they usually have letters printed on the back. For pilgrimage of the sacred place of Kannnon, 南無観世音菩薩 (Namu Kanzeon Bosatsu) is used. For henro (pilgrimage), it is usually 南無大師遍照金剛 "Namu Daishi Henjo Kongo" that is hogo (to chant the name of Kanzeon Bosatsu and bow repeatedly) of Kobo Daishi (Kukai). Sometimes depending on the sect, 南無阿弥陀仏 "Namu Amida Buddha" or 南無妙法蓮華経 "Nam-myoho-renge-kyo" is used.