TL;DR: Don't put steel strings on a classical guitar. 

X-bracing and fan bracing are two common bracing patterns used in the construction of acoustic guitars, and they are associated with different types of guitars — steel-string acoustic guitars often use X-bracing, while classical guitars typically feature fan bracing. 

X-bracing is commonly used in steel-string acoustic guitars, which are designed to handle the higher tension of steel strings, and involves two braces that cross each other in an "X" shape below the soundhole. This bracing pattern helps support the top of the guitar and reinforces it against the higher tension of steel strings. The placement and angles of the braces are carefully designed to achieve a balance between structural stability and resonance. 

X-bracing tends to provide a more pronounced and focused midrange, with a good balance between bass and treble frequencies. It is often associated with a powerful and projecting sound, making it suitable for styles that require a strong and cutting tone. 

Fan bracing is a traditional bracing pattern used in classical guitars, which are designed for lower tension  gut (or nylon) strings. The braces radiate out from the base of the guitar in a pattern that resembles a fan, with the braces being generally lighter and more delicate compared to the braces in X-braced guitars. Fan bracing is designed to allow the top of the guitar to vibrate more freely, producing a warm and rich tone. 

Fan-braced classical guitars typically have a broader tonal palette with a strong emphasis on the low end and a warm, mellow sound. The design aims to maximize sustain and resonance, producing a well-balanced and nuanced tone that complements the expressive requirements of classical and fingerstyle playing. 

While X-bracing and fan bracing represent two major categories, there are variations and hybrids within each, and luthiers may experiment with different bracing patterns to achieve specific tonal characteristics. The choice between X-bracing and fan bracing often depends on the type of strings the guitar will use, the desired tonal qualities, and the playing style for which the instrument is intended.