After defining the message type, a list of fields are given. A field is comprised of a repetition, a type, and the field name. Available repetitions are required, optional, and repeated.
Each field has a type. The primitive types include:

binary -used for strings

fixed_len_byte_array - used for byte arrays of fixed length

boolean - a 1 bit boolean value

int32 - a 32 bit integer

int64 - a 64 bit integer

int96 - a 96 bit integer

float - a 32 bit floating point number

double - a 64 bit floating point number

These types can be annotated with a logical type to specify how the application should interpret the data. The Logical types include:

UTF8 - used with binary to specify the string as UTF8 encoded

INT_8 - used with int32 to specify the int as an 8 bit signed integer

INT_16 - used with int32 to specify the int as a 16 bit signed integer

Unsigned types - may be used to produce smaller in-memory representations of the data. If the stored value is larger than the maximum allowed by int32 or int64, then the behavior is undefined.

UINT_8 - used with int32 to specify the int as an 8 bit unsigned integer

UINT_16 - used with int32 to specify the int as a 16 bit unsigned integer

UINT_32 - used with int32 to specify the int as a 32 bit unsigned integer

UINT_64 - used with int64 to specify the int as a 64 bit unsigned integer

DECIMAL(precision, scale) - used to describe arbitrary-precision signed decimal numbers of the form value * 10^(-scale) to the given precision. The annotation can be with int32, int64, fixed_len_byte_array, binary. See the Parquet documentation for limits on precision that can be given.

DATE - used with int32 to specify the number of days since the Unix epoch, 1 January 1970

Note: This Snap supports only the following date format: yyyy-MM-dd.

TIME_MILLIS - used with int32 to specify the number of milliseconds after midnight

TIMESTAMP_MILLIS - used with int64 to store the number of milliseconds from the Unix epoch, 1 January 1970

INTERVAL - used with a fixed_len_byte_array of length 12, where the array stores 3 unsigned little-endian integers. These integers specify

a number in months

a number in days

a number in milliseconds

JSON - used with binary to represent an embedded JSON document

BSON - used for an embedded BSON document

message document {
  # Primitive Types
  optional int64 32_num;
  optional int64 64_num;
  optional boolean truth;
  optional binary message;
  optional float pi;
  optional double e;
  optional int96 96_num;
  optional fixed_len_byte_array (1) one_byte;
  # Logical Types
  optional binary snowman (UTF8);
  optional int32 8_num (INT_8);
  optional int32 16_num (INT_16);
  optional int32 u8_num (UINT_8);
  optional int32 u16_num (UINT_16);
  optional int32 u32_num (UINT_32);
  optional int64 u64_num (UINT_64);
  optional int32 dec_num (DECIMAL(5,2));
  optional int32 jan7 (DATE);
  optional int32 noon (TIME_MILLIS);
  optional int64 jan7_epoch (TIMESTAMP_MILLIS);
  optional binary embedded (JSON);
}

"Generate template" will not work for nested structure like MAP and LIST type.