Placeables
Placeables are source strings (numbers, words, phrases etc.) that can be simply placed in the translation.
Strings a CAT tool might suggest as placeablesIn the strict sense, placeables are strings that are copied verbatim into the translation, for example numbers, proper names, tags or codes, etc. Some placeables may need to be edited, though, for example you need to replace 1,250,000 with 1 250 000 if you translate English to French or Romanize Владимир Путин if you translate Russian to English. CAT tools can usually help you with such placeable conversions.
In the broader sense, placeables may refer to any string your CAT tool lets you place in the translation, typically placeables in the strict sense plus recognized terms.
Placeables in Wordfast Pro
Wordfast Pro uses the term placeable exclusively to refer to its formatting tags: {ut1}, {ut2}, etc.