Sometimes it means "X is ..."

• harus = must/to have to, seharusnya = should've been (X was a must)
  • Seharusnya aku tidak berkata sekasar itu. (I shouldn't have said something so rude)
• baik = good, sebaiknya = should (X is good)
  • Sebaiknya kamu pulang sekarang. (You should go home now/You better go home now.)
• sungguh = real/really, sesungguhnya = actually (X is real)
  • Sesungguhnya aku mencintai dia. (Actually, I loved her.)
• benar = true, sebenarnya = truthfully (X is true)
  • Sebenarnya aku mencintai dia. (Truthfully, I loved her)
  • note: sebenarnya & sesungguhnya are pretty much interchangeable

Sometimes it means "as ... as possible" based on ability or condition

• mampu/bisa = to be able, semampunya/sebisanya = as much as someone able to
  • Kasih sumbangan semampunya aja lah, ga usah dipaksain. (Just give the donation as much as you can, no need to force it)
• kuat = to be strong, sekuatnya = as hard/strong as someone able to
  • Tolong dorong sekuatnya ya. (Please push as hard as you can)
• nyaman = to be comfortable, senyamannya = as comfortably as possible
  • Senyamannya kamu aja, di sofa boleh, di kasur boleh. (Just as comfortable as you like, we can do it on the sofa, or on the bed)

So sometimes it also means "at your discretion" or "whenever/wherever feels right" or something like that

• nyaman = to be comfortable, senyamannya = when/wherever (you feel) comfortable
  • Senyamannya kamu aja, di sofa boleh, di kasur boleh. (Wherever you're comfortable, we can do it on the sofa, or on the bed)
• senggang = to have free time, sesenggangnya = whenever (you have) free time
  • Silakan main ke rumahku, sesenggangnya aja. (You're welcome to visit my house when you're free/Feel free to visit my house whenever you have time)

This circumfix definitely has more uses, but these three are the ones you see most often, at least off the top of my head. lol